[CI] Fix false positive in database migration

- This also means that if one of the test fails, it will actually
propagate to make and subsequently fail the test.
- Remove the 'delete duplicates issue users' code, I checked this
against my local development database (which contains quite bizarre
cases, even some that Forgejo does not like), my local instance database
and against Codeberg production and they all yielded no results to this
query, so I'm removing it thus resolving the error that the delete code
was not compatible with Mysql.
- Sync all tables that are requires by the migration in the test.
- Resolves #2206

(cherry picked from commit 8e02be7e89)

models/migrations/fixtures/Test_AddCombinedIndexToIssueUser/issue_user.yml

@@ -11,10 +11,3 @@
   issue_id: 1
   is_read: true
   is_mentioned: false
-
--
-  id: 3
-  uid: 2
-  issue_id: 1 # duplicated with id 2
-  is_read: false
-  is_mentioned: true