Intermittent rbt_insert_and_remove unit test failures

One failure observed so far in https://gitlab.isc.org/isc-projects/bind9/-/jobs/33413:

lib/dns/tests/rbt_test:rbt_insert_and_remove  ->  failed: 2 checks failed; see output for more details  [9.538s]

Test output:

*** Check failed: rbt_test.c:962: names_count + 1 != dns_rbt_nodecount(mytree): line:1031: 2 != 1
*** Check failed: rbt_test.c:962: names_count + 1 != dns_rbt_nodecount(mytree): line:1041: 1 != 0

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