Improve instance-level and group-level feature test coverage of integrations

About

We have very few instance-level and group-level feature tests of integrations.

This exposes us to a lot of risk of introducing bugs like #331262 (closed).

Proposal

We should write some feature specs for instance-level and group-level integrations.

Initially, the feature specs could be basic, using a shared_examples group similar to 'project service activation' to assert that integrations can be saved correctly.

See !62054 (merged) which has introduced a basic framework for writing instance and group-level feature tests of integrations.