lacp: Stop time to avoid dependency on real time in negotiation test.

Commit f3103dfa00d (lacp.at: Change timing of lacp - negotiation test)
fixed test failures for the lacp negotiation test on some systems, but
made it crop up on others where it hadn't appeared before.

My guess is that this commit is the real fix.  If it isn't, then we
will probably just have to remove the test or disable it, much as I
hate doing that.

Signed-off-by: Ben Pfaff <blp@nicira.com>