rrl tests fail intermittently

We have one occurrence here:

• https://gitlab.isc.org/isc-projects/bind9/-/jobs/5453

S:rrl:Mon Mar 19 16:25:54 UTC 2018
T:rrl:1:A
A:rrl:System test rrl
I:rrl:PORTRANGE:11400 - 11499
I:rrl:0 instead of 50 '192.0.2.8' responses for a*.a9.tld2
I:rrl:60 instead of 10 dropped responses for a*.a9.tld2
I:rrl:0 instead of 50 NOERROR responses for a*.a9.tld2
I:rrl:exit status: 1
R:rrl:FAIL
E:rrl:Mon Mar 19 16:26:13 UTC 2018