selftests/bpf: Add tests for ring__avail_data_size

Add test for the new API ring__avail_data_size.

Signed-off-by: Martin Kelly <martin.kelly@crowdstrike.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230925215045.2375758-9-martin.kelly@crowdstrike.com