[9.18] Resolve "Notify source port test is not reliable"

Backport of MR !7509 (merged)

Send the test message from ns3 to ns2 instead of ns2 to ns3 as ns2 is started first and therefore the test doesn't have to wait on the resend of the the NOTIFY message to be successful.

(cherry picked from commit e7e1f59a)

Closes #3857 (closed)