[libcxx] [test] Add return values after assert(false) in some experimental tests

When building in MSVC mode (in release mode), the assert(false) don't
make the end of the function unreachable, so add return statements to
silence compiler warnings (treated as errors).

Also change 'virtual' into 'override', which was requested in review,
as these files require C++11.

Differential Revision: https://reviews.llvm.org/D99214