selftests: bpf: move sub-register zero extension checks into subreg.c

It is better to centralize all sub-register zero extension checks into an
independent file.

This patch takes the first step to move existing sub-register zero
extension checks into subreg.c.

Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>