tests: Fix test broken by introduction of ONFOXM_ET_TCP_FLAGS.

Commit 847ddeab372220 (meta-flow: Add support for ONF OpenFlow 1.3
extension ONFOXM_ET_TCP_FLAGS.) failed to update a test to match the new
OXM extension.  This fixes the problem.

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