[libc++] [test] Detect an improperly noexcept'ed __decay_copy.

`__decay_copy` is used by `std::thread`'s constructor to copy its arguments
into the new thread. If `__decay_copy` claims to be noexcept, but then
copying the argument does actually throw, we'd call std::terminate instead
of passing this test. (And I've verified that adding an unconditional `noexcept`
to `__decay_copy` does indeed fail this test.)

Differential Revision: https://reviews.llvm.org/D100277