Don't treat debug messages as errors

[Pawka]

Teaching gotestsum to recognize Golang cache debug output. When gocachehash=1 or gocachetest=1 debug options are enabled, output is sent to stderr. gotestum treats unrecognised stderr messages as errors what makes test suite run marked as failed.

Example of testing gotestsum repository without this PR. Each debug info line is treated as error:

$ GODEBUG=gocachehash=1,gocachetest=1 ./gotestsum
...
DONE 183 tests, 2 skipped, 13819 errors in 0.726s

After fixing the issue:

$ GODEBUG=gocachehash=1,gocachetest=1 ./gotestsum
...
DONE 183 tests, 2 skipped in 0.523s

[dnephin]

This is great! Thank you for the fix.

Do you know if there is any other way to enable this debug output? (ex: a flag to go build).

If this env var is the only way to enable it maybe we should add a check for the env var?

if os.Getenv("GODEBUG") == "" {
    return false
}

A test for this, something similar to TestScanOutput_WithNonJSONLines, would also be great.

[dnephin]

Thinking about it more, I don't think we need to guard on the env var because this is only scanning stderr, which we expect to be empty most of the time.

I added the test in a second commit.

[Pawka]

Thanks for merge! Sorry for not picking this up earlier - I was a bit distracted other things (summer, etc.).