selftests/bpf: Change NAME_NTH/IS_NAME_NTH for BTF_KIND_TAG format

BTF_KIND_TAG ELF format has a component_idx which might have value -1.
test_btf may confuse it with common_type.name as NAME_NTH checkes
high 16bit to be 0xffff. Change NAME_NTH high 16bit check to be
0xfffe so it won't confuse with component_idx.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210914223041.248009-1-yhs@fb.com