[evlatests] Bug in executor

[Barry Clark]

There is a race condition in Executor which primarily affects scans
or subscans of length five seconds.  It can occur in scans of length
between five and ten seconds.  It should not occur for scans shorter
than five seconds or longer than ten seconds.  If the race is won by
the wrong thread, extra pointing, and for VLA, delay commands are sent,
whose effects range from harmless to wrong pointing/delay for the entire
scan.

In the simulator, the race went to the wrong thread about 15% of the
time with five second scans and about 1.5% of the time with seven
second scans (your mileage may vary).

I am currently testing a fix in the simulator, will commit it tomorrow.