Add more comprehensive test coverage for the unifex::blocking CPO

https://github.com/facebookexperimental/libunifex/pull/652 fixed a typo in await_transform.hpp. Our tests probably missed it because we mostly use that CPO at compile time, which, in that case, doesn't need to compile the body because the function signature specifies a return type.

If we add better test coverage for the unifex::blocking CPO, issues like that would be caught earlier.