Add integration tests for add-on

[andyw8]

Closes https://github.com/Shopify/team-ruby-dx/issues/1327

This PR adds an integration test for the flow of generating DSL RBIs for a particular compiler.

I chose to use the ActiveJob compiler to avoid any database complexity.

I've tried to reduce the Rails files to the minimum possible.

[andyw8]

Looking into the test failures, seems unrelated to this PR though:

    it compile RBIs with location from gem source                   FAIL (1.20s)
        --- expected
        +++ actual
        @@ -1,66 +1,66 @@
        -"# source://the-default-gem//lib/bar.rb#1
        +"# source://the-default-gem/lib/bar.rb#1

[andyw8]

(I will squash before merging).
DONE

[vinistock]

This is a job, but it's under app/models. Is that intentional?

[andyw8]

@vinistock everything should have been addressed now.

[andyw8]

(squashed)