Add integration tests related to extending a config file from a non-reviewpad repository

[shay2025]

Is your feature request related to a problem? Please describe.
There are a few test cases described here https://www.notion.so/reviewpad/Extend-configuration-from-an-unauthorized-repository-85fbe0b78ace4ebe8062b283e9ef7d76 related to the handling of the error returned when the user tries to extend a reviewpad configuration that lives in a repository that doesn't have the Reviewpad GitHub App installed. However, doing these tests manually to test if there were regressions can be cumbersome.

Describe the solution you'd like
Implement integration tests that reflect the test cases mentioned above.