CI: Revisit Rails testing, syncing up with the latest Rails testing guide content

[fallwith]

Our agent predates much of the great quality Rails guide content available now, and our Rails related tests were seemingly authored by borrowing testing methodology directly from the Rails code base and leveraging things that are internally private to Rails and subject to change without public notice.

We should strive to modernize our Rails testing and see how far we can get with the latest Rails testing guide and see if we can move away from any private API usage or hacks that might no longer be necessary.

https://guides.rubyonrails.org/testing.html

[workato-integration[bot]]

https://issues.newrelic.com/browse/NR-169193

[workato-integration[bot]]

https://new-relic.atlassian.net/browse/NR-169193