selftests: forwarding: Add tc-police tests

Test tc-police action in various scenarios such as Rx policing, Tx
policing, shared policer and police piped to mirred. The test passes
with both veth pairs and loopbacked ports.

# ./tc_police.sh
TEST: police on rx                                                  [ OK ]
TEST: police on tx                                                  [ OK ]
TEST: police with shared policer - rx                               [ OK ]
TEST: police with shared policer - tx                               [ OK ]
TEST: police rx and mirror                                          [ OK ]
TEST: police tx and mirror                                          [ OK ]

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>