test: add cmdtest package (#15251)

## Description

This PR introduces the `cmdtest` package, offering a lightweight wrapper around cobra commands to simplify testing CLI utilities.

I backfilled tests for the `version` command, which was an example of a very simple test setup; and for the `export` command, which was more involved due to the server and client context requirements.

I did notice that there are some existing tests for some utilities, but the `cmdtest` package follows a simple pattern that has been easy to use successfully in [the relayer](https://github.com/cosmos/relayer/blob/main/internal/relayertest/system.go) and in other projects outside the Cosmos ecosystem.

While filling in these tests, I started removing uses of `cmd.Print`, as that is the root cause of issues like #8498, #7964, #15167, and possibly others. Internal to cobra, the print family of methods write to `cmd.OutOrStderr()` -- meaning that if the authors call `cmd.SetOutput()` before executing the command, the output will be written to stdout as expected; otherwise it will go to stderr. I don't understand why that would be the default behavior, but it is probably too late to change from cobra's side.

Instead of `cmd.Print`, we prefer to `fmt.Fprint(cmd.OutOrStdout())` or `fmt.Fprint(cmd.ErrOrStderr())` as appropriate, giving an unambiguous destination for output. And the new tests collect those outputs in plain `bytes.Buffer` values so that we can assert their content appropriately.

In the longer term, I would like to deprecate and eventually remove the `testutil` package's `ApplyMockIO` method and its `BufferWriter` and `BufferReader` types, as they are unnecessary indirection when a simpler solution exists. But that can wait until `cmdtest` has propagated through the codebase more.

---

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version/command.go

@@ -2,6 +2,7 @@ package version
 
 import (
 	"encoding/json"
+	"fmt"
 	"strings"
 
 	"github.com/cometbft/cometbft/libs/cli"
@@ -16,11 +17,12 @@ func NewVersionCommand() *cobra.Command {
 	cmd := &cobra.Command{
 		Use:   "version",
 		Short: "Print the application binary version information",
+		Args:  cobra.NoArgs,
 		RunE: func(cmd *cobra.Command, _ []string) error {
 			verInfo := NewInfo()
 
 			if long, _ := cmd.Flags().GetBool(flagLong); !long {
-				cmd.Println(verInfo.Version)
+				fmt.Fprintln(cmd.OutOrStdout(), verInfo.Version)
 				return nil
 			}
 
@@ -42,7 +44,7 @@ func NewVersionCommand() *cobra.Command {
 				return err
 			}
 
-			cmd.Println(string(bz))
+			fmt.Fprintln(cmd.OutOrStdout(), string(bz))
 			return nil
 		},
 	}

version/version_test.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"runtime"
+	"strings"
 	"testing"
 
 	"github.com/cometbft/cometbft/libs/cli"
@@ -11,6 +12,7 @@ import (
 	"github.com/stretchr/testify/require"
 
 	"github.com/cosmos/cosmos-sdk/testutil"
+	"github.com/cosmos/cosmos-sdk/testutil/cmdtest"
 	"github.com/cosmos/cosmos-sdk/version"
 )
 
@@ -40,6 +42,103 @@ go version go1.14 linux/amd64`
 	require.Equal(t, want, info.String())
 }
 
+func TestCLI(t *testing.T) {
+	setVersionPackageVars(t)
+
+	sys := cmdtest.NewSystem()
+	sys.AddCommands(version.NewVersionCommand())
+
+	t.Run("no flags", func(t *testing.T) {
+		res := sys.MustRun(t, "version")
+
+		// Only prints the version, with a newline, to stdout.
+		require.Equal(t, testVersion+"\n", res.Stdout.String())
+		require.Empty(t, res.Stderr.String())
+	})
+
+	t.Run("--long flag", func(t *testing.T) {
+		res := sys.MustRun(t, "version", "--long")
+
+		out := res.Stdout.String()
+		lines := strings.Split(out, "\n")
+		require.Contains(t, lines, "name: testchain-app")
+		require.Contains(t, lines, "server_name: testchaind")
+		require.Contains(t, lines, `version: "3.14"`)
+		require.Contains(t, lines, "commit: abc123")
+		require.Contains(t, lines, "build_tags: mybuildtag")
+
+		require.Empty(t, res.Stderr.String())
+	})
+
+	t.Run("--output=json flag", func(t *testing.T) {
+		res := sys.MustRun(t, "version", "--output=json")
+
+		var info version.Info
+		require.NoError(t, json.Unmarshal(res.Stdout.Bytes(), &info))
+
+		// Assert against a couple fields that are difficult to predict in test
+		// without copying and pasting code.
+		require.NotEmpty(t, info.GoVersion)
+
+		// The SDK version appears to not be set during this test, so we'll ignore it here.
+
+		// Now clear out the non-empty fields, so we can compare against a fixed value.
+		info.GoVersion = ""
+
+		want := version.Info{
+			Name:      testName,
+			AppName:   testAppName,
+			Version:   testVersion,
+			GitCommit: testCommit,
+			BuildTags: testBuildTags,
+		}
+		require.Equal(t, want, info)
+
+		require.Empty(t, res.Stderr.String())
+	})
+
+	t.Run("positional args rejected", func(t *testing.T) {
+		res := sys.Run("version", "foo")
+		require.Error(t, res.Err)
+	})
+}
+
+const (
+	testName      = "testchain-app"
+	testAppName   = "testchaind"
+	testVersion   = "3.14"
+	testCommit    = "abc123"
+	testBuildTags = "mybuildtag"
+)
+
+// setVersionPackageVars temporarily overrides the package variables in the version package
+// so that we can assert meaningful output.
+func setVersionPackageVars(t *testing.T) {
+	t.Helper()
+
+	var (
+		origName      = version.Name
+		origAppName   = version.AppName
+		origVersion   = version.Version
+		origCommit    = version.Commit
+		origBuildTags = version.BuildTags
+	)
+
+	t.Cleanup(func() {
+		version.Name = origName
+		version.AppName = origAppName
+		version.Version = origVersion
+		version.Commit = origCommit
+		version.BuildTags = origBuildTags
+	})
+
+	version.Name = testName
+	version.AppName = testAppName
+	version.Version = testVersion
+	version.Commit = testCommit
+	version.BuildTags = testBuildTags
+}
+
 func Test_runVersionCmd(t *testing.T) {
 	cmd := version.NewVersionCommand()
 	_, mockOut := testutil.ApplyMockIO(cmd)