libstdc++: Add stream state tests for chrono operator<<

Add tests covering formatted output function semantics for the
chrono operator<< overloads. These verify that the new stack-buffer
implementation correctly preserves sentry construction (badbit check
and tied stream flush), width/fill padding with all three alignment
modes (right, left, internal), width reset after each output, and
locale-aware output.  A weekday_indexed content test and a wchar_t
smoke test are included.

libstdc++-v3/ChangeLog:

	* testsuite/std/time/ostream_insert.cc: New test.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
Signed-off-by: Anlai Lu <agicy@qq.com>