selftests/bpf: ensure we always have non-ambiguous sorting in veristat

Always fall back to unique file/prog comparison if user's custom order
specs are ambiguous. This ensures stable output no matter what.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20221103055304.2904589-5-andrii@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>