[9.16] chg: test: Be more patient when stopping servers in the system tests

When the TCP test is run on the busy server, the server might take a
while to wind the server down because it might still be processing all
that 300k invalid XFR requests.

Increate the rncd wait time to 120 seconds, the SIGTERM time to 300
seconds, and reduce the time to wait for ans servers from 1200 second
to just 120 seconds.

(cherry picked from commit d97147232157a87ccb155b2d9c312eb7d1a21b4f)

Backport of MR !6847

Merge branch 'backport-ondrej-increase-the-time-to-wait-for-servers-to-gracefully-shutdown-9.16' into 'bind-9.16'

See merge request isc-projects/bind9!9468