Reduce 2015-em tests to 100 events

The 2015-em-rdst.qmt and 2015-em.qmt tests were running over 1000 events. This was introduced in 2015 to check in the nightlies that the memory usage was under control and no large memory leaks were introduced.

We now have other tools to check this (e.g. the sanitizers nightly) but also we don't care about regressions in the 2015 reconstruction, at best this test should be moved to the upgrade configuration.

Reducing to 100 events brings these two tests in line with other tests in terms of execution time.

@sponce FYI