tests: rename testcommon module to shared_test_code

This name always confuses me, since there is also test_common.py. And this
module is not actually a test suite, even though it starts with "test".
This also makes for better tab completion, e.g.

python3 -m unittest tests/te[Tab] -> tests/test_

tests/test_build.py

@@ -9,7 +9,7 @@ import yaml
 from pathlib import Path
 from unittest import mock
 
-from .testcommon import TmpCwd, mkdtemp
+from .shared_test_code import TmpCwd, mkdtemp
 
 import fdroidserver.build
 import fdroidserver.common