Resolve "Fix the DoH quota stress test on FreeBSD 11"

The merge request fixes a problem introduced after merging !5036 (merged) which makes the doth system test fail on FreeBSD 11:

I:doth:checking server quotas for both encrypted and unencrypted HTTP (71)
Traceback (most recent call last):
  File "/builds/isc-projects/bind9/bin/tests/system/doth/stress_http_quota.py", line 242, in <module>
    sys.exit(main())
  File "/builds/isc-projects/bind9/bin/tests/system/doth/stress_http_quota.py", line 234, in main
    run_test(http_secure=True)
  File "/builds/isc-projects/bind9/bin/tests/system/doth/stress_http_quota.py", line 223, in run_test
    assert multidig.alive(), "Multiple DIG instances are expected to be alive"
AssertionError: Multiple DIG instances are expected to be alive

It failed only on FreeBSD 11 (which is not a part of merge request-triggered pipelines, but is a part of daily scheduled ones).

The merge request changes the system tests' in question logic in such a way that makes it work more reliably. The problem on FreeBSD 11 was solved by setting the tcp-initial-timeout option to a larger value. The test on CI was running slow enough on FreeBSD 11 for the TCP idle timeout to kick in and break the tests' logic expectations.

Closes #2809 (closed)