re #8324 add _EVENT instruments to ISIS facilites.xml
re #8324 make unit test more flexible to the number of intruments
re #8324 make unit test more flexible to the number of diffractometers
Re #8324. Add an optional listener attribute to the livedata element.

This accounts for the valid possibility that a particular instrument uses
a different live listener to the facility default.
Merge remote-tracking branch 'origin/feature/8324_Event_instruments_in_Facilities'