selftests/bpf: switch BPF_ANNOTATE_KV_PAIR tests to BTF-defined maps

Switch tests that already rely on BTF to BTF-defined map definitions.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>