tests: add coverage for models migration helpers (#18162)

They were previously not covered at all, either by integration tests or unit tests.

This PR also fixes a bug where the `num_comments` field was incorrectly set to include all types of comments.

It sets num_closed_issues: 0 as default in milestone unit test fixtures. If they are not set, Incr("num_closed_issues") will be a noop because the field is null.

models/fixtures/milestone.yml

@@ -5,6 +5,7 @@
   content: content1
   is_closed: false
   num_issues: 1
+  num_closed_issues: 0
 
 -
   id: 2
@@ -13,6 +14,7 @@
   content: content2
   is_closed: false
   num_issues: 0
+  num_closed_issues: 0
 
 -
   id: 3
@@ -21,6 +23,7 @@
   content: content3
   is_closed: true
   num_issues: 1
+  num_closed_issues: 0
 
 -
   id: 4
@@ -29,6 +32,7 @@
   content: content random
   is_closed: false
   num_issues: 0
+  num_closed_issues: 0
 
 - 
   id: 5
@@ -37,3 +41,4 @@
   content: for testing with PRs
   is_closed: false
   num_issues: 0
+  num_closed_issues: 0