tests/sources/git.py: Testing the git:unlisted-submodule warning

 o Test that it is not triggered in show before fetch, because we
   don't know about the unlisted submodules yet

 o Test that it is triggered by a fetch command

 o Test that it is triggered by `show` after having completed a
   fetch command, since now we have the repository and know about
   the unlisted submodule

 o Test all of this under warning or error conditions (parameterized
   for fatal-warnings)