This adds 4 tests which test the new external commands feature:

* ExternalCommand - Tests the expected usage of external command invoked
  via cabal-install
* ExternalCommandSetup - Tests that the ./Setup interface does not
  support external commands (#9403)
* ExternalCommandEnv - Tests that environment variables are set and
  preserved appropiately  (#9402)
* ExternalCommandHelp - Test that `cabal help <cmd>` is interpreted appropiately (#9404)