Option to control whether stdout/stderr output is shown

`gotestsum` suppresses app stdout logging, but I would like to optionally see it. I tested all `--format` options and only `--format standard-verbose` prints stdout currently.

I'd suggest a new option like `--show-stdout` and possibly `--show-stderr` to control whether these streams are shown or not and this should work independently from the `--format` option.

Start by tracing gotestsum's handling of test stdout/stderr and the existing --format standard-verbose behavior. Define --show-stdout and --show-stderr as independent of --format, then verify that each stream is shown or suppressed as requested across the available formats.

Written by the indexing model from the issue text.