test: add API test for signature inheritance on OCI index children (#23707)

Covers the inherited_accessories field added in #23540. The test signs an
index with cosign and with notation, then checks that a child of that index
reports both signatures as inherited with the index digest as their subject,
while owning no accessory of its own and returning nothing extra on a request
that does not ask for them.

notation cert generate-test refuses to overwrite an existing key, so
generate_cert takes a common name and this test uses one of its own instead of
colliding with test_notation_sign_artifact.py in the same suite.

Signed-off-by: Ikram Aloui <ikram@container-registry.com>
Co-authored-by: Ikram Aloui <ikram@container-registry.com>
Co-authored-by: Prasanth Baskar <prasanth@8gears.com>
Co-authored-by: Wang Yan <wangyan_0219@hotmail.com>