stp: Fix sporadic failure in TC_ipa_to_m3ua_ni

It was spotted that from time to time the test set NI international
instead of national. This is presumable due to a race condition between
VTY and messages being received and forwarded.
Let's delay resetting the VTY config after the message is recieved, to
avoid the race conditions between them.

Change-Id: I3ef08447d2e36b6948d3db4ec9d1459beebc9384
1 file changed