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_removeInvalidLabels/label.yml

@@ -0,0 +1,26 @@
+# type Label struct {
+#   ID     int64 `xorm:"pk autoincr"`
+#   RepoID int64 `xorm:"INDEX"`
+#   OrgID  int64 `xorm:"INDEX"`
+# }
+-
+  id: 1
+  repo_id: 1
+  org_id: 0
+-
+  id: 2
+  repo_id: 0
+  org_id: 1
+-
+  id: 3
+  repo_id: 2
+  org_id: 0
+-
+  id: 4
+  repo_id: 1
+  org_id: 0
+-
+  id: 5
+  repo_id: 3
+  org_id: 0
+