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/issue.yml

@@ -0,0 +1,21 @@
+# type Issue struct {
+#   ID     int64 `xorm:"pk autoincr"`
+#   RepoID int64 `xorm:"INDEX UNIQUE(repo_index)"`
+#   Index  int64 `xorm:"UNIQUE(repo_index)"` // Index in one repository.
+# }
+-
+  id: 1
+  repo_id: 1
+  index: 1
+-
+  id: 2
+  repo_id: 2
+  index: 1
+-
+  id: 3
+  repo_id: 1
+  index: 2
+-
+  id: 4
+  repo_id: 3
+  index: 1