selftest: Move C-specific tests to c_family

selftest: Move C-specific tests to c_family

When trying to make use of the selftest framework over on the rust
frontend, we ran into issues where rust1 was expected to produce errors
containing C-like type names such as `int`.

I had gotten in contact with David Malcolm on the gcc mailing list [1],
who advised moving some test functions to a better location. The
offending functions have also been renamed in order to better fit the C
family of tests, and are thus not called when performing general
selftests anymore.

Kindly,

[1]: https://gcc.gnu.org/pipermail/gcc/2021-November/237703.html

2022-02-16 Arthur Cohen <arthur.cohen@embecosm.com>

gcc/c-family/

	* c-common.cc (c_family_tests): Call the new tests.
	* c-common.h (c_diagnostic_tests): Declare.
	(c_opt_problem_cc_tests): Likewise.

gcc/
	* diagnostic.cc (diagnostic_cc_tests): Rename to...
	(c_diagnostic_cc_tests): ...this.
	* opt-problem.cc (opt_problem_cc_tests): Rename to...
	(c_opt_problem_cc_tests): ...this.
	* selftest-run-tests.cc (selftest::run_tests): No longer run
	opt_problem_cc_tests or diagnostic_cc_tests.
	* selftest.h (diagnostic_cc_tests): Remove declaration.
	(opt_problem_cc_tests): Likewise.