selftests/bpf: Add BTF sanitize test covering BTF layout

selftests/bpf: Add BTF sanitize test covering BTF layout

Add test that fakes up a feature cache of supported BPF
features to simulate an older kernel that does not support
BTF layout information.  Ensure that BTF is sanitized correctly
to remove layout info between types and strings, and that all
offsets and lengths are adjusted appropriately.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Link: https://lore.kernel.org/r/20260408165735.843763-3-alan.maguire@oracle.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>