selftests/bpf: Add tests for ring__*_pos

Add tests for the new APIs ring__producer_pos and ring__consumer_pos.

Signed-off-by: Martin Kelly <martin.kelly@crowdstrike.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230925215045.2375758-7-martin.kelly@crowdstrike.com