statschannel test intermittently fails with incorrect zone loadtime

After #3983 (closed) was fixed and the loadtime check was re-enabled, the following tests occasionally fail because the loadtime retrieved over stats channel is 0.

• statschannel/tests_xml.py::test_zone_timers_secondary_xml
• statschannel/tests_json.py::test_zone_timers_secondary_json

________________________ test_zone_timers_secondary_xml ________________________
[gw1] linux -- Python 3.11.2 /usr/bin/python3
/builds/isc-projects/bind9/bin/tests/system/statschannel/tests_xml.py:118: in test_zone_timers_secondary_xml
    generic.test_zone_timers_secondary(
/builds/isc-projects/bind9/bin/tests/system/statschannel/generic.py:101: in test_zone_timers_secondary
    check_zone_timers(loaded, expires, refresh, mtime)
/builds/isc-projects/bind9/bin/tests/system/statschannel/generic.py:56: in check_zone_timers
    check_loaded(loaded, loaded_exp, now)
/builds/isc-projects/bind9/bin/tests/system/statschannel/generic.py:45: in check_loaded
    assert loaded == expected
E   assert datetime.datetime(2023, 12, 6, 0, 28, 43) == datetime.datetime(1970, 1, 1, 0, 0)