Fix Twilio phone number form validation test

[alanparmenter]

What github issue is this PR for, if any?

No Issue. Change too small to require one, right? Last of the failing tests on main in my local environment...

What changed, and why?

Brittle browser form validation message assertion replaced with testing browser form validity state (:invalid). This to avoid locale-dependent test failures due to "Fill in" (UK English) and "Fill out" (USA English) inconsistency.

How is this tested? (please write rspec and jest tests!) 💖💪

NB UK locale.
Before:

Failures:

1) casa_org/edit requires Twilio Form to be filled in correctly
    Failure/Error: expect(message).to eq "Please fill out this field."
     
       expected: "Please fill out this field."
            got: "Please fill in this field."
     
       (compared using ==)

After:
Passes with "" as a phone number.
Fails with "01234567":

 # Capybara::ExpectationNotMet:
     #   expected to find css "#casa_org_twilio_phone_number:invalid" but there were no matches

Screenshots please :)

Feelings gif (optional)

[compwron]

nice nice nice