selftests: net: add tc flower cfm test

New cfm flower test case is added to the net forwarding selfttests.

Example output:

 # ./tc_flower_cfm.sh p1 p2
 TEST: CFM opcode match test                                         [ OK ]
 TEST: CFM level match test                                          [ OK ]
 TEST: CFM opcode and level match test                               [ OK ]

Signed-off-by: Zahari Doychev <zdoychev@maxlinear.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>