output from TestMain is silently swallowed in GitHub Actions

[josharian]

I have important logging to stderr in TestMain that triggers only in rare scenarios.

But gotestsum silently changes formats in CI to a format that doesn't display any output from TestMain, only from the tests that it triggers.

This has caused...quite a lot of confusion and wasted time.

I would like to suggest that output from TestMain in GitHub Actions be grouped under a TestMain heading and made visible.

Given the many hours I've lost to this, I'm motivated to try my hand at a PR, if one would be welcome.