selftests: forwarding: Convert until_counter_is() to take expression

until_counter_is() currently takes as an argument a number and the
condition holds when the current counter value is >= that number. Make the
function more generic by taking a partial expression instead of just the
number.

Convert the two existing users.

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