fix: isolate every test module however it runs

tests/__init__.py keeps a run offline, its scratch under tmp/ and its
installer cache its own, but only discover -s tests -t . imported it.
discover tests ran with RETROBIOS_OFFLINE unset, scratch on the /tmp
tmpfs and the user's real installer cache, the setting in which e2e
fixtures had written into the shared .cache/large. Every test module now
imports the package first, and a test fails on a module that does not.
test_verify_listing.py took its line with the previous commit.

tests/test_split_pack.py

@@ -14,6 +14,8 @@ parts extracted into one folder are the pack.
 
 from __future__ import annotations
 
+import tests  # noqa: F401  offline, scratch under tmp/, the suite's own cache
+
 import contextlib
 import io
 import os