capture actual test warnings

capture actual test warnings

Compare expected `use warnings` output to warnings produced by similar code
instead of hardcoding text.  It doesn't really matter what is said, as long
it's the same using plp.

Fixes (so far and afaik) a slight change introduced by Perl v5.11.0 in
"Useless use of a constant in void context" (adding details about which
constant).