アップデートの前後の値比較
trigger UpdateTesObject on TesObject__c (before update) {
    for(TesObject__c tesObject : Trigger.New) {
        TesObject__c oldtesObject = Trigger.oldMap.get(tesObject.id);
        if (tesObject.TestStatus__c != oldtesObject.TestStatus__c ) {
            tesObject.Is_TestUpdated__c = true;
        }
    }
}