mkosi: Unify environment for unit tests

Some testcases in test-execute rely on existence of user groups with
certain gids. However, their existence is not universally granted [1].
Although the test could be skipped in their absence and give up testing
some code paths, different approach was chosen -- create dummy groups in
the testing image.

[1] See how systemd-sysusers allocates gids (e.g.
src/test/test-uid-range.c).