Factor out function to check for clang in tests

Factor out function to check for clang in tests

This also stops a flymake test from failing on my machine.

* lisp/emacs-lisp/ert-x.el (ert-gcc-is-clang-p): New function
factored out from ...
* test/lisp/progmodes/flymake-tests.el (flymake-tests--gcc-is-clang):
* test/lisp/cedet/semantic/bovine/gcc-tests.el
(semantic-gcc-test-output-parser-this-machine): ... here.

* test/lisp/progmodes/flymake-tests.el (different-diagnostic-types)
(included-c-header-files):
* test/lisp/cedet/semantic/bovine/gcc-tests.el
(semantic-gcc-test-output-parser-this-machine): Use above new function.