Add some api integration tests (#18872)

depends on #18871

Added some api integration tests to help testing of #18798.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

models/fixtures/milestone.yml

@@ -6,6 +6,7 @@
   is_closed: false
   num_issues: 1
   num_closed_issues: 0
+  completeness: 0
   deadline_unix: 253370764800
 
 -
@@ -16,6 +17,7 @@
   is_closed: false
   num_issues: 0
   num_closed_issues: 0
+  completeness: 0
   deadline_unix: 253370764800
 
 -
@@ -26,6 +28,7 @@
   is_closed: true
   num_issues: 1
   num_closed_issues: 0
+  completeness: 0
   deadline_unix: 253370764800
 
 -
@@ -36,14 +39,16 @@
   is_closed: false
   num_issues: 0
   num_closed_issues: 0
+  completeness: 0
   deadline_unix: 253370764800
 
-- 
+-
   id: 5
   repo_id: 10
-  name: milestone of repo 10 
+  name: milestone of repo 10
   content: for testing with PRs
   is_closed: false
   num_issues: 0
   num_closed_issues: 0
+  completeness: 0
   deadline_unix: 253370764800