test/ringbuf-read: Delete `.ringbuf-read.%d` before exit

Running test/ringbuf-read.t leaves untracked files in git status:

  Untracked files:
    (use "git add <file>..." to include in what will be committed)
          .ringbuf-read.163521
          .ringbuf-read.163564
          .ringbuf-read.163605
          .ringbuf-read.163648

Make sure we unlink it properly. While in there, fix the exit code,
use T_EXIT_*.

v2:
  - Use T_EXIT_* for exit code (comment from Alviro).

Cc: Alviro Iskandar Setiawan <alviro.iskandar@gnuweeb.org>
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Reviewed-by: Alviro Iskandar Setiawan <alviro.iskandar@gnuweeb.org>
Link: https://lore.kernel.org/r/20220905093126.376009-1-ammar.faizi@intel.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>