selftest/bpf: Add a test case for ld_imm64 copy logic.

Add a test case to exercise {btf_id, btf_obj_fd} copy logic between ld_imm64 insns.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230319203014.55866-2-alexei.starovoitov@gmail.com