Fix the `upforwd` system test
After the RCU changes were merged, the upforwd test started
consistenly failing when run under thread sanitizer. After some
investigation, it turned out that retry attempts were continuing after
the "update forwarding to dead primary" test. This caused mismatches
in the DNSTAP message counts for the subsequent tests, because they
were also counting retries.
Fix this problem by waiting for the nsupdate processes to exit.
While investigating the bug, I replaced several fixed 15 second delays
with wait_for_log, so the test runs faster.
Closes #4069 (closed)