selftests: forwarding: lib: Add tc_rule_handle_stats_get()

The function tc_rule_stats_get() fetches a given statistic of a TC rule
given the rule preference. Another common way to reference a rule is using
its handle. Introduce a dual to the aforementioned function that gets a
statistic given rule handle.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Amit Cohen <amitc@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>