libstdc++: Improve std::async test

libstdc++: Improve std::async test

Add tests for waiting for the future using both chrono::steady_clock and
chrono::system_clock in preparation for dealing with those clocks
properly in futex.cc.

libstdc++-v3/ChangeLog:

	* testsuite/30_threads/async/async.cc (test02): Test steady_clock
	with std::future::wait_until.
	(test03): Add new test templated on clock type waiting for future
	associated with async to resolve.
	(main): Call test03 to test both system_clock and steady_clock.