add more tests

core/stores/mongo/internal/collection.go

@@ -0,0 +1,17 @@
+//go:generate mockgen -package internal -destination collection_mock.go -source collection.go
+package internal
+
+import "github.com/globalsign/mgo"
+
+type MgoCollection interface {
+	Find(query interface{}) *mgo.Query
+	FindId(id interface{}) *mgo.Query
+	Insert(docs ...interface{}) error
+	Pipe(pipeline interface{}) *mgo.Pipe
+	Remove(selector interface{}) error
+	RemoveAll(selector interface{}) (*mgo.ChangeInfo, error)
+	RemoveId(id interface{}) error
+	Update(selector, update interface{}) error
+	UpdateId(id, update interface{}) error
+	Upsert(selector, update interface{}) (*mgo.ChangeInfo, error)
+}