## 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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Prepare migrating testing auxiliary functions from tests
to testutil.
Remove local duplicates on testutil.WriteToNewTempFile().
Always favor testutil.NewTestCaseDir() over ioutil.TempDir().
Add test cases for the testing auxiliary functions.
