Tests for GitHub.Sync.installation_event/1

[zacck-zz]

What's in this PR?

Adding test coverage for the GitHub.Sync.installation_event/1

References

Fixes #1373

Progress on: #

[zacck-zz]

Struggling to test some validation clauses, however, seeking some help testing for the following clauses

{:error, :validating_github_issue, Changeset.t()}
{:error, :validating_user, Changeset.t()}
{:error, :multiple_issue_users_match}

[zacck-zz]

Ok done!!

[zacck-zz]

@begedin ok ✂️ done!

[begedin]

@zacck As usual, thank you for your time, work and expertise. 👍