test/e2e: fix "podman load localhost registry from dir" test

When loading from an OCI directory without an explicit image name, the
image is now stored by digest instead of using the path as the name.

Update the test to verify the load succeeded and output contains the
sha256 digest, rather than checking for a specific image name.

This aligns with the behavior change from containers/common and fixes
the rootful localintegration test failure.

(partial cherry-pick from commit e66c04c1f7ae7851d94b5991209dc0d9251e3446)

Signed-off-by: Lokesh Mandvekar <[email protected]>