Fix race condition in kasp system test

The following discussion from !6315 (merged) should be addressed:

• @aram started a discussion: (+1 comment)

  @matthijs I just removed the ~"LGTM (Merge OK)" because of a suspicious failure in the kasp system test.

  https://gitlab.isc.org/isc-projects/bind9/-/jobs/2517061

  I don't see how this MR could have caused it, so if you also think that it's unrelated please let me know.

A timing issue: The test checks for "all zones loaded" to ensure the new zone content is loaded. But this is the unsigned zone, the signed zone still needs to be produced. The dig requests comes in before that process has finished.