selftests/bpf: Test libbpf API function btf__add_tag()

Add btf_write tests with btf__add_tag() function.

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/20210914223036.247560-1-yhs@fb.com