Print warnings caused by tests to stderr

If they are caught and we raise RuntimeError those warnings will not be
visible anymore, which makes noticing and fixing them so much harder.

Author: mweinelt

tests/async_wrapper.py

@@ -1,5 +1,6 @@
 import asyncio
 import warnings
+import sys
 
 
 
@@ -11,6 +12,9 @@ def _await(coro):
         ret = loop.run_until_complete(coro)
         loop.close()
 
+        for warn in warns:
+            print(warn.message, file=sys.stderr)
+
         if warns:
             raise RuntimeError