libstdc++: Skip time zone format testing for COW std::string

libstdc++: Skip time zone format testing for COW std::string

This is needed when testing with -D_GLIBCXX_USE_CXX11_ABI=0 to fix:
FAIL: std/time/format/empty_spec.cc  -std=gnu++20 (test for excess errors)

libstdc++-v3/ChangeLog:

	* testsuite/std/time/format/empty_spec.cc: Only test time zones
	for cxx11 string ABI.