[libcxx] [test] Use LIBCPP_ASSERT for libc++-specific checks

This makes it a little easier to reuse the libc++ tests for another
std::lib (specifically libstdc++).

The regex_error::what() strings are different in other
implementations, so should be tested with LIBCPP_ASSERT so that those
checks are skipped for other implementations.

The value of ECMAScript constant is non-zero in other implementations,
and conditionally for libc++, so adjust the preprocessor condition for
that too.

Reviewed By: ldionne, #libc, philnik, Mordante

Spies: Mordante, philnik, libcxx-commits

Differential Revision: https://reviews.llvm.org/D122574