bridge: Add test that ports that disappear get added back to the datapath.

The test added in this commit would have caught the bug fixed by commit
96be8de595150 (bridge: When ports disappear from a datapath, add them
back.).  With that commit reverted, the new test fails.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Gurucharan Shetty <gshetty@nicira.com>