Use Mocha vs Minitest Mock in inviting users test

The use of MiniTest::Mock in this test was causing a problem when trying
to upgrade to Minitest v5.19.0, because we should be using the newer
Minitest module name.

I could have just changed that here, but it seemed odd that we're using
Minitest mocking & stubbing in this one test, while in every other test
we're using Mocha. So I decided to convert this test to use Mocha too.

I'm pretty confident that we don't need to "expect" the calls to the
code & body methods on the response since their values are read and
asserted against in the assert_response_contains call, so using stubbed
return values is sufficient.

I think these changes also make the test easier to read.

test/integration/inviting_users_test.rb

@@ -346,22 +346,16 @@ class InvitingUsersTest < ActionDispatch::IntegrationTest
     end
 
     should "raise an error if email address is not in Notify team" do
-      raises_exception = lambda do |_request, _params|
-        response = MiniTest::Mock.new
-        response.expect :code, 400
-        response.expect :body, "Can't send to this recipient using a team-only API key"
-        raise Notifications::Client::BadRequestError, response
-      end
+      response = stub("response", code: 400, body: "Can't send to this recipient using a team-only API key")
+      User.stubs(:invite!).raises(Notifications::Client::BadRequestError, response)
 
-      User.stub(:invite!, raises_exception) do
-        perform_enqueued_jobs do
-          visit new_user_invitation_path
-          fill_in "Name", with: "Fred Bloggs"
-          fill_in "Email", with: "[email protected]"
-          click_button "Create user and send email"
+      perform_enqueued_jobs do
+        visit new_user_invitation_path
+        fill_in "Name", with: "Fred Bloggs"
+        fill_in "Email", with: "[email protected]"
+        click_button "Create user and send email"
 
-          assert_response_contains "Error: One or more recipients not in GOV.UK Notify team (code: 400)"
-        end
+        assert_response_contains "Error: One or more recipients not in GOV.UK Notify team (code: 400)"
       end
     end
   end