fix: test: Fix intermittent multisigner system test failures

The multisigner system test could query an inline-signed zone
before the signed view had processed an update, intermittently
observing the previous DNSKEY, CDNSKEY, or CDS RRset. Record
the served SOA serial before each same-server update and wait
for it to advance before checking the results.

Merge branch 'ondrej/fix-the-multisigner-tests-timing' into 'main'

See merge request isc-projects/bind9!12573