selftests/bpf: Use explicit bpf_test_load_program() helper calls

Remove the second part of prog loading testing helper re-definition:

  -Dbpf_load_program=bpf_test_load_program

This completes the clean up of deprecated libbpf program loading APIs.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Dave Marchevsky <davemarchevsky@fb.com>
Link: https://lore.kernel.org/bpf/20211103220845.2676888-13-andrii@kernel.org