tests: Fix test failure in "ofproto-dpif - NetFlow flow expiration".

Commit 89b5ad6f43b (netflow: Fix interpretation of flow_seq.) fixed a bug
in the NetFlow code, which caused a failure in a test that depended on the
broken behavior.  This commit fixes up the test's expected output to match
the new, correct output.

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