[clang] tests: cleanup, update and add some new ones

This reworks a small set of tests, as preparatory work for implementing
P2266.
* Run for more standard versions, including c++2b.
* Normalize file names and run commands.
* Adds some extra tests.

New Coroutine tests taken from Aaron Puchert's D68845.

Signed-off-by: Matheus Izvekov <mizvekov@gmail.com>

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D99225