selftests/bpf: Add btf_field_iter selftests

The added selftests verify that for every BTF kind we iterate correctly
over consituent strings and ids.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20240605153314.3727466-1-alan.maguire@oracle.com