ofproto.at: Fix races in rule eviciton tests

ofproto.at: Fix races in rule eviciton tests

Bump timeout differences, because timeouts different by 1s might end up
to have the same position in the heap as rule_eviction_priority() uses
1024ms as a unit.

Also, use time/stop to avoid relying on how long an add-flow would take.

These tests were introduced by commit 6d56c1f1.
("ofproto: Update rule's priority in eviction group.")

Acked-by: Ben Pfaff <blp@nicira.com>
Acked-by: Kmindg G <kmindg@gmail.com>
Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>