Fix tests that fail with _GLIBCXX_USE_CXX11_ABI=0

The pmr::string typedef isn't available for the old COW string, so some
tests that use it were failing.

	* testsuite/20_util/variant/run.cc: Use a new Hashable type to test
	hashing, because pmr::string depends on _GLIBCXX_USE_CXX11_ABI==1.
	* testsuite/21_strings/basic_string/hash/hash.cc
	[!_GLIBCXX_USE_CXX11_ABI]: Don't test pmr strings.
	* testsuite/21_strings/basic_string/hash/hash_char8_t.cc
	[!_GLIBCXX_USE_CXX11_ABI]: Likewise.

From-SVN: r270604