timing issues with rndc system test

Job #1940981 failed for aae53e21:

The test does not wait for the post freeze zone writes to complete.

I:rndc:checking update (68)
I:rndc:rndc freeze
I:rndc:edit zone files
I:rndc:rndc thaw
I:rndc:rndc reload
I:rndc:ns7 server reload successful
I:rndc:checking zone file edits are loaded (69)
I:rndc:failed
I:rndc:exit status: 1

grep '\(test/IN\|'"'"'\(reload\|freeze\|thaw\)'"'"'\|all zones loaded\)' ns7/named.run
...
25-Aug-2021 00:27:36.398 received control channel command 'freeze'
25-Aug-2021 00:27:36.398 zone_dump: zone test/IN/internal: enter
25-Aug-2021 00:27:36.398 freezing zone 'test/IN' internal: success
25-Aug-2021 00:27:36.398 zone_gotwritehandle: zone test/IN/internal: enter
25-Aug-2021 00:27:36.430 received control channel command 'thaw'
25-Aug-2021 00:27:36.434 zone test/IN/internal: starting load
25-Aug-2021 00:27:36.434 zone_startload: zone test/IN/internal: enter
25-Aug-2021 00:27:36.434 thawing zone 'test/IN' internal: success
25-Aug-2021 00:27:36.442 dump_done: zone test/IN/internal: enter
25-Aug-2021 00:27:36.442 zone_journal_compact: zone test/IN/internal: target journal size 336
25-Aug-2021 00:27:36.442 zone test/IN/internal: dns_journal_compact: success
25-Aug-2021 00:27:36.442 zone_loaddone: zone test/IN/internal: enter
25-Aug-2021 00:27:36.442 zone test/IN/internal: number of nodes in database: 4
25-Aug-2021 00:27:36.442 zone test/IN/internal: loaded; checking validity
25-Aug-2021 00:27:36.442 dns_zone_verifydb: zone test/IN/internal: enter
25-Aug-2021 00:27:36.442 zone test/IN/internal: ixfr-from-differences: unchanged
25-Aug-2021 00:27:36.442 zone_postload: zone test/IN/internal: done
25-Aug-2021 00:27:36.466 received control channel command 'reload'
...