feat(soak): add warmup and stats

Specifically on 2n-icx, soak tests are suffering
from the first search trial (at half of max rate)
reporting unsent packets, thus polluting the results, see CSIT-1867.
This adds a plain trial (no stats, result ignored), fixing the issue.

To keep the structure similar to NDRPDR tests,
stats trial is done after the search, at the discovered critical rate.

Ticket: CSIT-1867

Change-Id: I12ff6f58c4eb174594e667bfa86ba1585164b8ef
Signed-off-by: Vratko Polak <vrpolak@cisco.com>