[RTADPA BW Tests] Check for env variable OVERRIDE_EVTMAX.

cc @rjhunter

Adds a check for an OVERRIDE_EVTMAX variable, else defaults to EVTMAX=1e5.

This is useful for at least the TCK infrastructure work, so that we can easily use the AllenInMoore_hlt1_bandwidth test over different number of events without modifying the Moore_bandwidth_test.sh directly.
(i.e. we can access the EVTMAX variable through the command line via

Moore/run env OVERRIDE_COMMIT=1e4 ...