Extend flymake's warning predicate to be a function. Test suite for flymake.

Extend flymake's warning predicate to be a function.  Test suite for flymake.
* lisp/progmodes/flymake.el (flymake-warning-predicate): New.
(flymake-parse-line): Use it.
(flymake-warning-re): Make obsolete alias to
`flymake-warning-predicate'.
* doc/misc/flymake.texi (Parsing the output, Customizable variables):
Add reference to `flymake-warning-predicate'.
* test/automated/flymake-tests.el:
* test/automated/flymake/warnpred/Makefile
* test/automated/flymake/warnpred/test.c
* test/automated/flymake/warnpred/test.pl: New files.

Fixes: debbugs:14217