[mypyc] Fix async mypyc tests on Windows (#19578)

This is part of fixing the failed tests in
#19545

The async tests previously used many invocations of `asyncio.run`, which
likely caused issues with event loop management. The documentation for
`asyncio.run` states:
> This function cannot be called when another asyncio event loop is
running in the same thread. ...
> This function should be used as a main entry point for asyncio
programs, and should ideally only be called once.

Calling `asyncio.run` multiple times could cause the test processes to
hang for strange event loop reasons. This commit converts most test
cases to be run in a single event loop managed by the default driver,
which is now async aware.

Not all tests could be converted, e.g. the test that runs an async
function in a sync context. However, the test suite does succeed with
these changes, and these tests can be further modified if needed.

Author: emmatyping

mypyc/test-data/driver/driver.py

@@ -9,6 +9,10 @@
 
 import sys
 import native
+import asyncio
+import inspect
+
+evloop = asyncio.new_event_loop()
 
 failures = []
 tests_run = 0
@@ -18,7 +22,10 @@
         test_func = getattr(native, name)
         tests_run += 1
         try:
-            test_func()
+            if inspect.iscoroutinefunction(test_func):
+                evloop.run_until_complete(test_func)
+            else:
+                test_func()
         except Exception as e:
             failures.append((name, sys.exc_info()))