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)

* wip to add test-code-generators field to test stanzas

* fixups

* change hashes

* regen golden parser test output

* docs and changelog

* test

* Update pr-7688

* tweak test

Co-authored-by: Gershom Bazerman <gershom@arista.com>