API: fix set milestone on PR creation (#14981)

* API: fix set milestone on PR creation

pr creation via API failed with 404, because we searched
for milestoneID 0, due to uninitialized var usage D:

* add tests

* fix expected status codes

* fix tests

Co-authored-by: 6543 <6543@obermui.de>

models/fixtures/milestone.yml

@@ -29,3 +29,11 @@
   content: content random
   is_closed: false
   num_issues: 0
+
+- 
+  id: 5
+  repo_id: 10
+  name: milestone of repo 10 
+  content: for testing with PRs
+  is_closed: false
+  num_issues: 0