Create Proper Migration Tests (#15116)

* Create Proper Migration tests

Unfortunately our testing regime has so far meant that migrations do not
get proper testing.

This PR begins the process of creating migration tests for this.

* Add test for v176

* fix mssql drop db

Signed-off-by: Andrew Thornton <art27@cantab.net>

models/migrations/fixtures/Test_deleteOrphanedIssueLabels/issue_label.yml

@@ -0,0 +1,29 @@
+# Issue_Label 1 should not be deleted
+-
+  id: 1
+  issue_id: 1
+  label_id: 1
+
+# Issue_label 2 should be deleted
+-
+  id: 2
+  issue_id: 5
+  label_id: 99
+
+# Issue_Label 3 should not be deleted
+-
+  id: 3
+  issue_id: 2
+  label_id: 1
+
+# Issue_Label 4 should not be deleted
+-
+  id: 4
+  issue_id: 2
+  label_id: 4
+
+-
+  id: 5
+  issue_id: 2
+  label_id: 87
+

models/migrations/fixtures/Test_deleteOrphanedIssueLabels/label.yml

@@ -0,0 +1,43 @@
+-
+  id: 1
+  repo_id: 1
+  org_id: 0
+  name: label1
+  color: '#abcdef'
+  num_issues: 2
+  num_closed_issues: 0
+
+-
+  id: 2
+  repo_id: 1
+  org_id: 0
+  name: label2
+  color: '#000000'
+  num_issues: 1
+  num_closed_issues: 1
+-
+  id: 3
+  repo_id: 0
+  org_id:  3
+  name: orglabel3
+  color: '#abcdef'
+  num_issues: 0
+  num_closed_issues: 0
+
+-
+  id: 4
+  repo_id: 0
+  org_id: 3
+  name: orglabel4
+  color: '#000000'
+  num_issues: 1
+  num_closed_issues: 0
+
+-
+  id: 5
+  repo_id: 10
+  org_id: 0
+  name: pull-test-label
+  color: '#000000'
+  num_issues: 0
+  num_closed_issues: 0