Add invite spec example to diagnose flakey example

The example just before this one keeps failing in the CircleCI
build [1], but I haven't been able to replicate it locally. It's hard
to work out what's going on, because `SchoolTeacher::Invite#call`
swallows any exception. I'm hoping that this new assertion _might_ fail
for the same reason, but then display the exception message to give us
more clues.

[1]: https://app.circleci.com/pipelines/github/RaspberryPiFoundation/editor-api/2359/workflows/ecf1c252-3f22-45e6-b0fe-08363f66c8d9/jobs/4447

Author: floehopper

spec/concepts/school_teacher/invite_spec.rb

@@ -16,6 +16,11 @@
     expect(response.success?).to be(true)
   end
 
+  it 'does not return an error in operation response' do
+    response = described_class.call(school:, school_teacher_params:, token:)
+    expect(response[:error]).to be_blank
+  end
+
   it 'creates a TeacherInvitation' do
     expect { described_class.call(school:, school_teacher_params:, token:) }.to change(TeacherInvitation, :count)
   end