[libc++][test] Replace a TEST_NOEXCEPT_FALSE with noexcept(false). NFC.

Replace `TEST_NOEXCEPT_FALSE` directly with `noexcept(false)` in
optional hash test which is only run in C++17 or later.
`TEST_NOEXCEPT_FALSE` is only useful in C++03 context where `noexcept`
isn't supported by clang. `TEST_NOEXCEPT_FALSE` now only has one remaining use
in `hash_unique_ptr.pass.cpp`.