Looks like the test doesn't delete its cookies after use, so they affect subsequent tests.
$ run-webkit-tests http/tests/cache/partitioned-cache.html http/tests/cookies/third-party-cookie-relaxing.html
Regressions: Unexpected text-only failures (1)
http/tests/cookies/third-party-cookie-relaxing.html [ Failure ]
Created attachment 209486 [details]
Comment on attachment 209486 [details]
Seems reasonable, though I'd really like jpfau to have a look.