chore: QoL improvements to tests (#7917)

- Use mock helper functions, instead of home-brew solutions.
- Disable cron jobs that are not important to be run during integration tests and might even interfere.
- Avoid sleeping unnecessary, if there's some requirement then sleep or retry until that requirement is met.
- Avoid trying to deliver webhooks that will always result in a failure.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7917
Reviewed-by: Michael Kriese <michael.kriese@gmx.de>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

models/repo/pushmirror_test.go

@@ -40,8 +40,6 @@ func TestPushMirrorsIterate(t *testing.T) {
 		Interval:       0,
 	})
 
-	time.Sleep(1 * time.Millisecond)
-
 	repo_model.PushMirrorsIterate(db.DefaultContext, 1, func(idx int, bean any) error {
 		m, ok := bean.(*repo_model.PushMirror)
 		assert.True(t, ok)