[PATCH] make lib/warnings.t use t/test.pl

I was about to add some more OS-specific fumblings to lib/warnings.t but
decided to go the other direction and relegate much of what was already
there to t/test.pl, which already handles such things. The test now
passes on VMS and still passes on Mac OS X.

• [PATCH] make lib/warnings.t use t/test.pl by Craig A. Berry
• Re: [PATCH] make lib/warnings.t use t/test.pl by Nicholas Clark
• [PATCH] follow-up to #23765 by Craig A. Berry
• Re: [PATCH] follow-up to #23765 by Rafael Garcia-Suarez