Replace assertResponse(), fix declaring ::setUp without a void typehint

Before work is done on fixing these, we need to add a Gitlab CI test to cover Drupal 8. I know that this is no longer supported, but we have committed to keeping the current 8.x-1.x dev version (and hence at least 8.x-1.5) compatible with D8, so we need test coverage before we start modifying the code.

The Gitlab test changes are in a separate commit, pushed to 8.x-1.x

Branch pipeline testing now looks like this:

All tests pass now.

Automatically closed - issue fixed for 2 weeks with no activity.