Add tests for lupus_decoupled_form

Test for user login form is now passing

this seems to miss some essential test - the actual login. I think we should add that.
also, this is only about user-forms atm, what is fine. Let's make one issue per test we add, to keep a better overview.

@fago we are already testing the logging in in https://git.drupalcode.org/project/lupus_decoupled/-/blob/1.x/modules/lupus_decoupled_user_form/tests/src/Functional/LupusSessionDomainFunctionalTest.php.

But since the main test prepares environment for testing the API output I did put the login form API output in the main module test. I'll move the endpoint testing to the form module.