SQLStore: Fix wrong usage of xorm's insert functions in tests (#63850)

* SQLStore: Fix InsertId

* Prefs: Fix Insert return value

* Fix tests

* Add guidelines

pkg/services/preference/prefimpl/store.go

@@ -9,6 +9,7 @@ import (
 type store interface {
 	Get(context.Context, *pref.Preference) (*pref.Preference, error)
 	List(context.Context, *pref.Preference) ([]*pref.Preference, error)
+	// Insert adds a new preference and returns its sequential ID
 	Insert(context.Context, *pref.Preference) (int64, error)
 	Update(context.Context, *pref.Preference) error
 	DeleteByUser(context.Context, int64) error