Kai, I strongly disagree.
Testing only proves that the circuit with those particular components work under those conditions that day. The only way to make a reliable thing is by design.

Don't insult me! I never stated that testing should be done INSTEAD of developing.
Reliability test is recommended for oscillator circuits, which were developed by carefully obeying instructions of according datasheet and application note, as it is the obligatory procedure for professionals. It's suggested to people, which carefully designed their oscillator circuit but still wonder, whether it will actually work under extreme conditions. And I named those critical extremes, namely limits of temperature range and long power-up time.