[9.18] Watch logs from start in dialup system test
When the first parametrized test takes a bit longer than usual, the zone transfer in ns3 may succeed before the second parametrized test is even started, and then watch_log_from_here() won't find the "Transfer status: success" message in the named log. Using watch_log_from_start() instead makes sure the test is more stable.
(cherry picked from commit 283a7ab1)
Backport of MR !8782 (merged).