test-lib: allow prefixing a custom string before "ok N" etc.

test-lib: allow prefixing a custom string before "ok N" etc.

This is not really meant for external use, and thus not documented. It
allows the next commit to neatly distinguish between sub-tests and the
main run.

The format is intentionally not valid TAP.  The use in the next commit
would not result in anything valid either way, and it seems better to
make it obvious.

Signed-off-by: Thomas Rast <redacted>
Signed-off-by: Junio C Hamano <redacted>