core: add more tests for stat/datetime guessing

tests/cli.py

@@ -0,0 +1,4 @@
+from subprocess import check_call
+
+def test_lists_modules() -> None:
+    check_call(['hpi', 'modules'])

tests/core.py

@@ -13,3 +13,4 @@ we can run against the tests in my.core directly.
 from my.core.core_config import *
 from my.core.error       import *
 from my.core.util        import *
+from my.core.common      import *