[coroutines] Add end-to-end tests within libc++
This patch adds end-to-end/breathing tests for coroutines
into libc++. The tests aren't specifically to test libc++ requirements
but instead are intented to ensure coroutines are working fine in general.
Although libc++ isn't exactly the most correct place for these tests
to live, there is one major advantage. The libc++ test suite is also
used by MSVC and by adding the tests here it ensures they will be
run against all currently available coroutine implementations.