selftests/bpf: Replace all uses of bpf_load_btf() with bpf_btf_load()

Switch all selftests uses of to-be-deprecated bpf_load_btf() with
equivalent bpf_btf_load() calls.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211209193840.1248570-10-andrii@kernel.org