Add test case for scriptlet behavior

Add test case for scriptlet behavior

- The test-root has a shell but rpm doesn't know about it, add a
  fakeshell metapackage that provides it. Should come in handy for
  other cases as well, up to now we've been limited to lua scripts only.
- Test that all the "normal" scripts get executed, in expected order
  and with expected arguments, on install, upgrade and erase.
- This would've saved the rather embarrassing breakage with commit
  274dbf557d1cac90f7c278f9b6d6af05997d92df at least, but better
  late than never as they say... It also serves to highlight and
  document  the arguably broken arguments to %pre- and %posttrans
  scripts (ie always 0)