reloads in ixfr system test happen too fast

The ixfr system test can fail because reloads happen too fast causing the reload of zone files to be skipped as up to date.

I:ixfr:testing 'request-ixfr yes' option inheritance from view (7)
I:ixfr:ns3 server reload successful
I:ixfr:exceeded time limit waiting for literal 'got incremental response' in ns4/named.run
I:ixfr:failed

05-Oct-2022 16:18:18.158 received control channel command 'reload'
05-Oct-2022 16:18:18.254 received control channel command 'reload'
05-Oct-2022 16:18:18.350 received control channel command 'reload'