TrigP1Test: Increase default number of events for "build" tests
Increase the default number of events for TrigP1Test "build" tests from 50 to 100, as agreed with @hartj, @jpanduro, @dzanzi. This has nearly no effect on the total test execution time, because majority of the time is anyway spent in athena initialisation. The event loop is quick for these tests.
The only exceptions are the "T0Mon" tests which include full offline reco, which is slow. For those, the old number of 50 events is kept.