Move test.test_support.catch_warning() to the warnings module, rename it

Move test.test_support.catch_warning() to the warnings module, rename it
catch_warnings(), and clean up the API.

While expanding the test suite, a bug was found where a warning about the
'line' argument to showwarning() was not letting functions with '*args' go
without a warning.

Closes issue 3602.
Code review by Benjamin Peterson.