tests: allow application/octet-stream as content-type for kernels
ftp1.scientificlinux.org started to serve kernels with a application/octet-stream content type, which is a generic content; accept it since it is acceptable.
$ curl --head -L 'http://ftp1.scientificlinux.org/linux/scientific/obsolete/6.6/x86_64/os/images/pxeboot/vmlinuz'
HTTP/1.1 200 OK
Server: nginx
Date: Tue, 14 May 2024 13:08:32 GMT
Content-Type: application/octet-stream
Content-Length: 4153072
Last-Modified: Tue, 09 Dec 2014 17:46:43 GMT
Connection: keep-alive
ETag: "54873583-3f5ef0"
Content-Security-Policy: default-src 'self'; img-src 'self' data:; style-src 'self' 'unsafe-inline'; script-src 'self' 'unsafe-inline';
Accept-Ranges: bytes