libstdc++: Test for formatting with empty spec for time points.

libstdc++: Test for formatting with empty spec for time points.

Adding a tests for behavior of the ostream operator and the formatting
with empty chrono-spec for the chrono types. Current coverage is:
 * time point, zoned_time and local_time_format in this commit,
 * duration and hh_mm_ss in r16-1099-gac0a04b7a254fb,
 * calendar types in r16-1016-g28a17985dd34b7.

libstdc++-v3/ChangeLog:

	* testsuite/std/time/format/empty_spec.cc: New tests.

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