diag.t: Validate severity of ck warn routines better

In the case of Perl_ck_warner and other _ck_ functions, we can tell
whether it is a severe/default warning or no by the _d suffix (or the
presence of ‘dep’ in the name for regcomp.c’s ckWARN* functions).

This caught one incorrect severity in perldiag.pod, which is also cor-
rected here.