Add tests for async and apply overloads that accept regular C++ functions

[brycelelbach]

This was on my TODO list, and I'm not sure if it's still relevant. A couple of weeks/months
ago, I was trying to use async/apply with regular C++ functions, and was surprised to find
that including hpx/apply.hpp gave a compile error. At the time, we had no test that pulled in
the overloads of async/apply for regular C++ functions. If that is still the case today, then we should
create said tests.