https://crrev.com/c/2972915 previously added more gitignore files to the
repository. This left the repo dirty after running mjsunit tests due to
lingering .log files.

- Add test/mjsunit/tools/tmp dir to keep and ignore temporary log files
  without the need for a platform specific tmp dir
- Use temporary logfiles with --logfile=+ for log-ci.js tests

Change-Id: I1b1a47f45603d6c3027c6ca7050c78e8df0664ce
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2992720Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75451}