fix(grep): put --max-len before positionals in bench; reuse assert_eq_grep

Address review:
- benchmark.sh: with trailing_var_arg on extra_args, --max-len after the
  positionals was swallowed and forwarded raw to grep (unknown option ->
  bench FAIL). Move it before -rn/pattern so rtk parses it; verified 320KB
  -> 14.8KB (~95%) and that --max-len 40 vs 200 changes output size.
- grep_faithful_format_test: the inline cmp closure duplicated the existing
  assert_eq_grep helper (rtk_grep + grep_plain, same stdout/exit asserts);
  drop it in.