Implent C++20 std::atomic_flag::test

        * include/bits/atomic_base.h (atomic_flag): Implement test member
        function.
        * include/std/version: Define __cpp_lib_atomic_flag_test.
        * testsuite/29_atomics/atomic_flag/test/explicit.cc: New file.
        * testsuite/29_atomics/atomic_flag/test/implicit.cc: New file.