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)