Instead of a single test that only checks whether <regex> can be
included in Parallel Mode, add tests for each of C++11/C++14/C++17 that
check whether <bits/extc++.h> is compatible with _GLIBCXX_PARALLEL.
This increases the coverage to (almost) all headers.

If <omp.h> is not available then the tests will trivially pass, because
we don't care about compatibility with _GLIBCXX_PARALLEL in that case.

	PR libstdc++/89023
	* testsuite/17_intro/headers/c++2011/parallel_mode.cc: New test.
	* testsuite/17_intro/headers/c++2014/parallel_mode.cc: New test.
	* testsuite/17_intro/headers/c++2017/parallel_mode.cc: New test.
	* testsuite/28_regex/headers/regex/parallel_mode.cc: Remove.

From-SVN: r268769