tests: asyncio: collect garbage in run_until_complete_nesting

The test checks that coro1 emits a runtime warning when it is garbage
collected. cpython will do this collection immediately, but other
implementations such as PyPy might not. Add the appropriate call to
support.gc_collect() to ensure the warning is emitted while still in the
with statement.

Author: benzea

Lib/test/test_asyncio/test_events.py

@@ -287,6 +287,7 @@ async def coro2():
         ):
             self.assertRaises(
                 RuntimeError, self.loop.run_until_complete, coro2())
+            support.gc_collect()
 
     # Note: because of the default Windows timing granularity of
     # 15.6 msec, we use fairly long sleep times here (~100 msec).