Add stdlib/@tests? #11762

There's a slight inconsistency that's been bugging me: Third-party stubs – sensibly – can have a @tests directory, which contains the stubtest allowlists and regression tests. On the other hand, the allowlists for the standard library are in tests/stubtest_allowlists and the test cases in test_cases/stdlib. (The test_cases directory is otherwise empty, except for a README file.) For consistency's sake and easier discoverability, I suggest we add a stdlib/@tests directory with the same structure as the corresponding stubs/*/@tests directories and move the existing files there.