[libcxx] [test] Fix a test error with condvars with trivial destruction

If the destructor is trivial (_LIBCPP_HAS_TRIVIAL_CONDVAR_DESTRUCTION,
the constructor always is), the compiler warns about the
std::condition_variable being unused.

Add a cast to void to silence the warning about the object being unused.

Differential Revision: https://reviews.llvm.org/D97540