Re #7656. Add an 'expected' file for our BASIS data file.

This checks the number of events in the first event list (and the second,
just for fun) to avoid any regression of the fix for the bug described
in ticket #7524.
Re #7656. Add test of filtered loading vs loading then filtering.

This replaces a (slow) unit test that was supposed to test the same thing.
Merge remote branch 'origin/feature/7656_speed_up_loadeventnexustest'