selftests: forwarding: lib: Allow reading TC rule byte counters

The function tc_rule_stats_get() fetches a packet counter of a given TC
rule. Extend it to support byte counters as well by adding an optional
argument with selector.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>