Repurpose test_run to test_relative_dates

See reasoning in changes.

tests/filter/test_run.py → tests/filter/test_relative_dates.py

@@ -1,3 +1,8 @@
+# This file contains functional tests that would normally be written as
+# Cram-style tests. Without an easy way to mock the time in Cram tests (which is
+# necessary to test features that utilize the system time), an alternative is to
+# write equivalent tests using pytest+freezegun.
+
 import argparse
 from freezegun import freeze_time
 import pytest