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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testutil/cmdtest/system.go

@@ -0,0 +1,120 @@
+// Package cmdtest contains a framework for testing cobra Commands within Go unit tests.
+package cmdtest
+
+import (
+	"bytes"
+	"context"
+	"io"
+
+	"github.com/spf13/cobra"
+)
+
+// System is a system under test.
+type System struct {
+	commands []*cobra.Command
+}
+
+// NewSystem returns a new System.
+func NewSystem() *System {
+	// We aren't doing any special initialization yet,
+	// but let's encourage a constructor to make it simpler
+	// to update later, if needed.
+	return new(System)
+}
+
+// AddCommands sets commands to be available to the Run family of methods on s.
+func (s *System) AddCommands(cmds ...*cobra.Command) {
+	s.commands = append(s.commands, cmds...)
+}
+
+// RunResult is the stdout and stderr resulting from a call to a System's Run family of methods,
+// and any error that was returned.
+type RunResult struct {
+	Stdout, Stderr bytes.Buffer
+
+	Err error
+}
+
+// Run calls s.RunC with context.Background().
+func (s *System) Run(args ...string) RunResult {
+	return s.RunC(context.Background(), args...)
+}
+
+// RunC calls s.RunWithInput with an empty stdin.
+func (s *System) RunC(ctx context.Context, args ...string) RunResult {
+	return s.RunWithInputC(ctx, bytes.NewReader(nil), args...)
+}
+
+// RunWithInput calls s.RunWithInputC with context.Background().
+func (s *System) RunWithInput(in io.Reader, args ...string) RunResult {
+	return s.RunWithInputC(context.Background(), in, args...)
+}
+
+// RunWithInputC executes a new root command with subcommands
+// that were set in s.AddCommands().
+// The command's stdin is set to the in argument.
+// RunWithInputC returns a RunResult wrapping stdout, stderr, and any returned error.
+func (s *System) RunWithInputC(ctx context.Context, in io.Reader, args ...string) RunResult {
+	rootCmd := &cobra.Command{}
+	rootCmd.AddCommand(s.commands...)
+
+	rootCmd.SetIn(in)
+
+	var res RunResult
+	rootCmd.SetOutput(&res.Stdout)
+	rootCmd.SetErr(&res.Stderr)
+
+	rootCmd.SetArgs(args)
+
+	res.Err = rootCmd.ExecuteContext(ctx)
+	return res
+}
+
+// MustRun calls s.Run, but also calls t.FailNow if RunResult.Err is not nil.
+func (s *System) MustRun(t TestingT, args ...string) RunResult {
+	t.Helper()
+
+	return s.MustRunC(t, context.Background(), args...)
+}
+
+// MustRunC calls s.RunWithInput, but also calls t.FailNow if RunResult.Err is not nil.
+func (s *System) MustRunC(t TestingT, ctx context.Context, args ...string) RunResult { //nolint:revive // As a variation of MustRun, t is more important than ctx.
+	t.Helper()
+
+	return s.MustRunWithInputC(t, ctx, bytes.NewReader(nil), args...)
+}
+
+// MustRunWithInput calls s.RunWithInput, but also calls t.FailNow if RunResult.Err is not nil.
+func (s *System) MustRunWithInput(t TestingT, in io.Reader, args ...string) RunResult {
+	t.Helper()
+
+	return s.MustRunWithInputC(t, context.Background(), in, args...)
+}
+
+// MustRunWithInputC calls s.RunWithInputC, but also calls t.FailNow if RunResult.Err is not nil.
+func (s *System) MustRunWithInputC(t TestingT, ctx context.Context, in io.Reader, args ...string) RunResult { //nolint:revive // As a variation of MustRun, t is more important than ctx.
+	t.Helper()
+
+	res := s.RunWithInputC(ctx, in, args...)
+	if res.Err != nil {
+		t.Logf("Error executing %v: %v", args, res.Err)
+		t.Logf("Stdout: %q", res.Stdout.String())
+		t.Logf("Stderr: %q", res.Stderr.String())
+		t.FailNow()
+	}
+
+	return res
+}
+
+// TestingT is a subset of testing.TB,
+// containing only what the (*System).Must methods use.
+//
+// This simplifies using other testing wrappers,
+// such as testify suite, etc.
+type TestingT interface {
+	Helper()
+
+	Logf(format string, args ...any)
+
+	FailNow()
+}