selftests/bpf: Update btf_dump__new() uses to v1.0+ variant

Update to-be-deprecated forms of btf_dump__new().

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211111053624.190580-8-andrii@kernel.org