[9.18] fix: test: Check exit status of dig and nsupdate in nsupdate system test

Add missing failure checks to six dig and nsupdate invocations in nsupdate system test so that command failures are properly caught instead of silently ignored.

Backport of MR !11811

Merge branch 'backport-marka/check-return-codes-in-nsupdate-test-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!11814