libstdc++: Improve tests for __cpp_lib_erase_if macro

libstdc++: Improve tests for __cpp_lib_erase_if macro

	* testsuite/21_strings/basic_string/erasure.cc: Check for updated
	value of __cpp_lib_erase_if.
	* testsuite/23_containers/deque/erasure.cc: Likewise.
	* testsuite/23_containers/forward_list/erasure.cc: Likewise.
	* testsuite/23_containers/list/erasure.cc: Likewise.
	* testsuite/23_containers/map/erasure.cc: Likewise.
	* testsuite/23_containers/set/erasure.cc: Likewise.
	* testsuite/23_containers/unordered_map/erasure.cc: Likewise.
	* testsuite/23_containers/unordered_set/erasure.cc: Likewise.
	* testsuite/23_containers/vector/erasure.cc: Likewise.