feat(ci): allow manual triggering of the test suite

When a skip ci label is added to the PR title, removing it won't
trigger the CI. There needs to be a way to manually trigger it.

.forgejo/workflows/testing.yml

@@ -6,6 +6,7 @@ on:
     branches:
       - 'forgejo*'
       - 'v*/forgejo*'
+  workflow_dispatch:
 
 jobs:
   backend-checks: