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/repository.yml

@@ -0,0 +1,17 @@
+# type Repository struct {
+#   ID        int64  `xorm:"pk autoincr"`
+#   OwnerID   int64  `xorm:"UNIQUE(s) index"`
+#   LowerName string `xorm:"UNIQUE(s) INDEX NOT NULL"`
+# }
+-
+  id: 1
+  owner_id: 2
+  lower_name: "repo1"
+-
+  id: 2
+  owner_id: 1
+  lower_name: "repo2"
+-
+  id: 3
+  owner_id: 2
+  lower_name: "repo3"