asyncio: Add basic loop.call_soon() test.

asyncio/test_call_soon.py

@@ -0,0 +1,13 @@
+import asyncio
+import time
+
+
+def cb():
+    print("callback")
+    time.sleep(0.5)
+    loop.call_soon(cb)
+
+
+loop = asyncio.get_event_loop()
+loop.call_soon(cb)
+loop.run_forever()