[libc++] Tidy up tests for deduction guides and other compile-time failing properties...

[libc++] Tidy up tests for deduction guides and other compile-time failing properties in std::string

Instead of using `.fail.cpp` tests, use `.verify.cpp` to check for the
exact reason of the failure. In the case of deduction guides, use SFINAE
based tests instead since that is our preferred way of testing those.

Finally, ensure that we actually run the test in `iter_alloc_deduction.pass.cpp`,
since we were not running anything before.

Differential Revision: https://reviews.llvm.org/D123055