The the test is marked as ImageOnlyFailure in LayoutTests/TestExpectations, but it passes many times in WebKitGTK. See: https://build.webkit.org/results/GTK%20Linux%2064-bit%20Release%20(Tests)/r261273%20(13650)/imported/w3c/web-platform-tests/fetch/http-cache/basic-auth-cache-test-diff.png
This test, or group of tests, has been passing for the last 4000 revisions. Closing bug. Gardened in r278519.