Warnings

An alarm of any sort is merely a signaling device, it does not make the danger go away. In order to survive, you and your household must take action. Make a fire escape plan, discuss it with everyone in your home (including children) and practice it.
No smoke detector can sound the alarm instantly. The fire will grow and spread before the alarm sounds. Consequently, when an alarm sounds, you MUST get yourself and everyone else in your home out as quickly as possible. In a house fire, the difference between death and survival is often measured in minutes; sometimes seconds.
Laws in your jurisdiction probably specify how one must dispose of outdated and unreliable smoke detectors. Check the rules that apply in your area, and dispose of old and unreliable detectors properly.
It is dangerous to test a smoke detector using flame. It is safer to use an aerosol test spray. However: Never try to test a smoke alarm with any aerosol spray other than one specifically manufactured for that purpose. Other types of sprays contain material that will stick to the sensor, and make the device less reliable in the future.
Do not use candles or incense to test a smoke detector. The smoke produced by candles and incense contain waxy or oily particles that can contaminate the sensor and reduce its sensitivity.
All the test button does is check the BATTERY. It does NOT check the smoke sensor.