linux/kernel/testing: 4.8-rc3 -> 4.8-rc4

Tested by only building the linux_testing attribute, but haven't yet
tested it in production.

I've also fixed the extraMeta.branch attribute.

Verified-with-PGP: ABAF 11C6 5A29 70B1 30AB E3C4 79BE 3E43 0041 1886
Signed-off-by: aszlig <aszlig@redmoonstudios.org>