fix: run the suite so its package setup applies

The documented command, discover tests, makes tests/ the top level,
so tests/__init__.py never ran and the variable it sets to keep tests
offline was missing from every local run. Every documented and CI
run now passes -s tests -t ., and a test requires it.

.github/workflows/validate.yml

@@ -156,7 +156,7 @@ jobs:
         # resolved. The download tests mock the network and clear it.
         env:
           RETROBIOS_OFFLINE: "1"
-        run: python -m unittest discover tests -v
+        run: python -m unittest discover -s tests -t . -v
 
   label-pr:
     runs-on: ubuntu-latest