improve system tests

- increase prefetch test timing tolerance.
- remove five-second pause and explicit connection closing in tcp test
  as they are no longer necessary.

bin/tests/system/tcp/tests.sh

@@ -166,12 +166,8 @@ check_stats_limit() {
 	assert_int_equal "${TCP_HIGH}" "${TCP_LIMIT}" "TCP high-water value" || return 1
 }
 retry 2 check_stats_limit || ret=1
-close_connections $((TCP_LIMIT + 1)) || :
 if [ $ret != 0 ]; then echo_i "failed"; fi
 status=$((status + ret))
 
-# wait for connections to close
-sleep 5
-
 echo_i "exit status: $status"
 [ $status -eq 0 ] || exit 1