diag.t: Fix FAIL and vFAIL support

diag.t: Fix FAIL and vFAIL support

Until recently, it was treating them like Perl_croak et al., expecting
there to be an aTHX_.

Now that I fixed that, it turns out that FAIL and vFAIL (which were
being ignored, since all the calls lacked aTHX_) never worked anyway.

If they are matched by $source_msg_call_re, they end up in the wrong
if/else branch; namely, the one for handling packWARN, and they skip
over the one for tacking ‘in regex’ on to the end of the message.