[TESTS] add unit test for repository collaboration (#825)
- Add a few extra test cases and test functions for the collaboration
model to get everything covered by tests (except for error handling, as
we cannot suddenly mock errors from the database).
```
-> % go tool cover -func=coverage.out | grep "code.gitea.io/gitea/models/repo/collaboration.go"
```
Before:
```
code.gitea.io/gitea/models/repo/collaboration.go:28: init 100.0%
code.gitea.io/gitea/models/repo/collaboration.go:39: GetCollaborators 61.5%
code.gitea.io/gitea/models/repo/collaboration.go:65: CountCollaborators 0.0%
code.gitea.io/gitea/models/repo/collaboration.go:70: GetCollaboration 0.0%
code.gitea.io/gitea/models/repo/collaboration.go:83: IsCollaborator 100.0%
code.gitea.io/gitea/models/repo/collaboration.go:87: getCollaborations 42.9%
code.gitea.io/gitea/models/repo/collaboration.go:102: ChangeCollaborationAccessMode 77.8%
code.gitea.io/gitea/models/repo/collaboration.go:141: IsOwnerMemberCollaborator 0.0%
```
After:
```
code.gitea.io/gitea/models/repo/collaboration.go:28: init 100.0%
code.gitea.io/gitea/models/repo/collaboration.go:39: GetCollaborators 61.5%
code.gitea.io/gitea/models/repo/collaboration.go:65: CountCollaborators 100.0%
code.gitea.io/gitea/models/repo/collaboration.go:70: GetCollaboration 100.0%
code.gitea.io/gitea/models/repo/collaboration.go:83: IsCollaborator 100.0%
code.gitea.io/gitea/models/repo/collaboration.go:87: getCollaborations 100.0%
code.gitea.io/gitea/models/repo/collaboration.go:102: ChangeCollaborationAccessMode 83.3%
code.gitea.io/gitea/models/repo/collaboration.go:141: IsOwnerMemberCollaborator 87.5%
```
Co-authored-by: Gusted <postmaster@gusted.xyz>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/825
(cherry picked from commit 9de2b618a4)
(cherry picked from commit e3a6ca67e4)
(cherry picked from commit ae57e1b6d7)
(cherry picked from commit 5e37ea475d)
ef9494ddef