Write tests for GitHub.Sync.pull_request_event/1

# Problem

`GitHub.Sync.pull_request_event/1` is currently completely untested. Since we want to unit test it's individual elements to, it should suffice to test each possible outcome of the transaction performed by the tests, as defined by `GitHub.Sync.pull_request_event_outcome()`

## Subtasks
- [x] write test for `{:ok, map}` outcome
- [x] write test for `{:error, :repo_not_found}` outcome
- [x] write test for `{:error, :fetching_issue, struct}` outcome
- [x] write test for `{:error, :validating_github_pull_request, Changeset.t()}` outcome
- [ ] write test for `{:error, :validating_github_issue, Changeset.t()}` outcome
- [ ] write test for `{:error, :validating_user, Changeset.t()}` outcome
- [ ] write test for `{:error, :multiple_issue_users_match}` outcome
- [x] write test for `{:error, :validating_task, Changeset.t()}` outcome
- [ ] write test for `{:error, :unexpected_transaction_outcome, any}` outcome