selftests/bpf: Don't truncate #test/subtest field

We currently expect up to a three-digit number of tests and subtests, so:

  #999/999: some_test/some_subtest: ...

Is the largest test/subtest we can see. If we happen to cross into
1000s, current logic will just truncate everything after 7th character.
This patch fixes this truncate and allows to go way higher (up to 31
characters in total). We still nicely align test numbers:

  #60/66   core_reloc_btfgen/type_based___incompat:OK
  #60/67   core_reloc_btfgen/type_based___fn_wrong_args:OK
  #60/68   core_reloc_btfgen/type_id:OK
  #60/69   core_reloc_btfgen/type_id___missing_targets:OK
  #60/70   core_reloc_btfgen/enumval:OK

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20231006175744.3136675-3-andrii@kernel.org