libstdc++: Remove redundancy in test pathnames
Repeating "explicit_instantiation" in these long pathnames is not
necessary.
libstdc++-v3/ChangeLog:
* testsuite/20_util/duration/requirements/explicit_instantiation/explicit_instantiation.cc:
Moved to...
* testsuite/20_util/duration/requirements/explicit_instantiation.cc: ...here.
* testsuite/20_util/time_point/requirements/explicit_instantiation/explicit_instantiation.cc:
Moved to...
* testsuite/20_util/time_point/requirements/explicit_instantiation.cc: ...here.
* testsuite/20_util/unique_ptr/requirements/explicit_instantiation/explicit_instantiation.cc:
Moved to...
* testsuite/20_util/unique_ptr/requirements/explicit_instantiation.cc: ...here.