fs-test.sh: minor fixes

- Re-direct stderr into the log files, so any errors U-Boot emits are
  visible in the logs. This is relevant if the "reset" shell command
  attempts to report that it's not supported on the sandbox board.
- Fix test_fs_nonfs() to name the files it created differently for each
  invocation. Otherwise, the logs from different tests overwrite
  each-other.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Acked-by: Suriyan Ramasami <suriyan.r@gmail.com>