test: capture stderr for tests

[phm07]

In the future this will be useful for testing "-o=json" on resource creation, because the actual JSON and status messages will be printed to stdout/stderr respectively.

[jooola]

Have you considered renaming the out var to stdout in this PR as well ?

You will probably end up with out, stderr, err := fx.Run(cmd, []string{}) so you might as well rename everything now to maintain consistency:

stdout, _, err := fx.Run(cmd, []string{})
stdout, stderr, err := fx.Run(cmd, []string{})

[phm07]

Have you considered renaming the out var to stdout in this PR as well ?

You will probably end up with out, stderr, err := fx.Run(cmd, []string{}) so you might as well rename everything now to maintain consistency:

stdout, _, err := fx.Run(cmd, []string{})
stdout, stderr, err := fx.Run(cmd, []string{})

In the vast majority of cases there won't be any output to stderr anyway so the second string can be safely ignored. In that case out is still a concise variable name in my opinion.
If you use the second return value though, I agree that the naming you suggested is sensible. I don't think renaming every out now would be needed though.