test: tuning integration test for OFPT_FLOW_REMOVED

tuning grace sec of tests for OFPT_FLOW_REMOVED.

Sometimes, we will received the OFPT_FLOW_REMOVED message before
rewirte the duration time of the ofp_flow_removed by the switch.

Signed-off-by: HIYAMA Manabu <hiyama.manabu@po.ntts.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>