tools/syz-testbed: respond 404 to favicon requests
tools/syz-testbed: respond 404 to favicon requests

Otherwise the page is requested two times by Chrome.

See https://bugs.chromium.org/p/chromium/issues/detail?id=39402

google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller