test/flow_classify: fix return types

UTs should return either TEST_SUCCESS or TEST_FAILED only.
They should not return 0, -1 and any other value.

Fixes: 9c9befea4f ("test: add flow classify unit tests")
CC: stable@dpdk.org
Signed-off-by: Reshma Pattan <reshma.pattan@intel.com>
Reviewed-by: Anatoly Burakov <anatoly.burakov@intel.com>