Refactor flymake-tests.el in preparation for more tests

Refactor flymake-tests.el in preparation for more tests

Introduce a slightly more generic fixture macro.

Also make flymake-tests.el friendlier to interactive runs, by not
killing buffers visited by the user.

* test/lisp/progmodes/flymake-tests.el
(flymake-tests--call-with-fixture): New helper from
flymake-tests--current-face.  Don't kill file buffers already
being visited before the test starts.
(flymake-tests--with-flymake): New macro.
(flymake-tests--current-face): Delete.
(warning-predicate-rx-gcc, warning-predicate-function-gcc)
(warning-predicate-rx-perl, warning-predicate-function-perl):
Use flymake-test--with-flymake.