Conformance Test: `References teardown` should expect 404 response

[northtyphoon]

After deleting the subject manifest, the registry can trigger cascade deletion and automatically delete the reference manifest. If the test tries to delete the reference manifest again, it should also expect 404 response.

1. Delete subject manifest
   

   distribution-spec/conformance/03_discovery_test.go

   Line 440 in 6bc8715

   req = client.NewRequest(reggie.DELETE, "/v2/<name>/manifests/<digest>", reggie.WithDigest(manifests[4].Digest))

2. Delete the reference manifest again
   

   distribution-spec/conformance/03_discovery_test.go

   Line 442 in 6bc8715

   req = client.NewRequest(reggie.DELETE, "/v2/<name>/manifests/<reference>",