You misunderstand the meaning of test, the UPS tests itself periodically as part of its normal operation. In this case, if it fails the test it takes your servers with it.

I imagine the test is to ensure the accuracy of the battery runtime prediction, it transfers your load off line power and onto the inverter & batteries. Unfortunately something happens that causes the UPS to shut down or pause rather than immediately switch back to line power. Often the only evidence is that your servers power-cycled unexpectedly, sometimes the UPS actually switches off entirely during the test, however.