chore(spanner): fix event loop leak in unit tests (#17343)

This PR resolves leaky asyncio event loops inside the Spanner unit tests
that were causing flaky results during local nox testing. The original
version of the event loop was not closing out events and we were left
with too many open files.

Fixes #17049

Author: chalmerlowe

packages/google-cloud-spanner/tests/unit/gapic/conftest.py

@@ -11,10 +11,14 @@ def provide_loop_to_sync_grpc_tests():
     If no global loop exists, `grpc.aio` engine crashes during initialization.
     """
     try:
-        loop = asyncio.get_event_loop()
+        asyncio.get_running_loop()
     except RuntimeError:
         loop = asyncio.new_event_loop()
         asyncio.set_event_loop(loop)
-
-    yield
-    # No close here, just ensure existance
+        try:
+            yield
+        finally:
+            loop.close()
+            asyncio.set_event_loop(None)
+    else:
+        yield