Try once more to fix sigtrap.t

Try once more to fix sigtrap.t

It produces output like this:
lib/sigtrap....................................................sh: line 1: 66151 Abort trap              ./perl "-I../lib" -Msigtrap=INT -e 'sub { kill q-INT-, $$ } -> (3)' 2>&1
ok

And is effectively skipped on Windows.

So this new revision of the test
• calls the signal handler directly, instead of trying to trigger it
  with a signal and
• overrides kill globally to stop sigtrap.pm from aborting.