It's pretty typical for TAP implementations to offer an interface for emitting diagnostic messages as TAP comments. Examples:

• Test::More (Perl)
• pgTAP (Postgres)
• Test.More (JavaScript)

I tried to figure out where to plug one in to submit a patch, but the path to the output handle was over my head, so I used console.log() for my immediate use. Would be nice to have a canonical place for it.