Remove tests.signals module

This module was support code for the `SaveSignalHandlingModel` tests.
That class got introduced in 5d6f8f4e and removed in 11f3a53b, but
the support code remained.

tests/signals.py

@@ -1,6 +0,0 @@
-def pre_save_test(instance, *args, **kwargs):
-    instance.pre_save_runned = True
-
-
-def post_save_test(instance, created, *args, **kwargs):
-    instance.post_save_runned = True