selftests: devlink_lib: Split out helper

Merely checking whether a trap counter incremented or not without
logging a test result is useful on its own. Split this functionality to
a helper which will be used by subsequent patches.

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