libstdc++: Avoid rounding errors on custom clocks in condition_variable

The fix for PR68519 in 83fd5e73b3c16296e0d7ba54f6c547e01c7eae7b only
applied to condition_variable::wait_for. This problem can also apply to
condition_variable::wait_until but only if the custom clock is using a
more recent epoch so that a small enough delta can be calculated. let's
use the newly-added chrono::__detail::ceil to fix this and also make use
of that function to simplify the previous wait_for fixes.

Also, simplify the existing test case for PR68519 a little and make its
variables local so we can add a new test case for the above problem.
Unfortunately, the test would have only started failing if sufficient
time has passed since the chrono::steady_clock epoch had passed anyway,
but it's better than nothing.

libstdc++-v3/ChangeLog:

	* include/std/condition_variable (condition_variable::wait_until):
	Convert delta to steady_clock duration before adding to current
	steady_clock time to avoid rounding errors described in PR68519.
	(condition_variable::wait_for): Simplify calculation of absolute
	time by using chrono::__detail::ceil in both overloads.
	* testsuite/30_threads/condition_variable/members/68519.cc:
	(test_wait_for): Renamed from test01. Replace unassigned val
	variable with constant false. Reduce scope of mx and cv
	variables to just test_wait_for function.
	(test_wait_until): Add new test case.

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