testsuite: asyncify the testsuite driver

This patch refactors the testsuite driver, gets rid of multi-threading
logic for running test cases concurrently, and uses asyncio &
coroutines instead. This is not yak shaving for its own sake; the
previous multi-threading logic is prone to livelock/deadlock
conditions for some reason, even if the total number of threads is
bounded to a thread pool's capacity.

The asyncify change is an internal implementation detail of the
testsuite driver and does not impact most GHC maintainers out there.
The patch does not touch the .T files, test cases can be
added/modified the exact same way as before.