libstdc++: Make std::atomic_flag::test members const

libstdc++: Make std::atomic_flag::test members const

Also fix the tests so they run without an explicit -std=gnu++2a in the
RUNTESTFLAGS, and test the new function on const-qualified objects.

	* include/bits/atomic_base.h (atomic_flag::test): Add missing
	const qualifiers.
	* testsuite/29_atomics/atomic_flag/test/explicit.cc: Add
	dg-options and verify results of test function.
	* testsuite/29_atomics/atomic_flag/test/implicit.cc: Likewise.