lacp.at: Change timing of lacp - negotiation test

The "lacp - negotiation" test (# 3) used to always fail when running
multiple tests in parallel (-j2).  The reduced simulated wait time
(from 4 seconds to 2.5 seconds) seems to still give the expected result,
and works also when running multiple tests in parallel.

Signed-off-by: Jarno Rajahalme <jarno.rajahalme@nsn.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>