selftests: bpf: Add a test for mmapable vmlinux BTF

Add a basic test for the ability to mmap /sys/kernel/btf/vmlinux.
Ensure that the data is valid BTF and that it is padded with zero.

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Alan Maguire <alan.maguire@oracle.com>
Link: https://lore.kernel.org/bpf/20250520-vmlinux-mmap-v5-2-e8c941acc414@isovalent.com