We run tests in parallel which can cause multiple tests to write to
the shared v8.log file. This obviously breaks the simple assertions in
mjsunit/tools/log.js.

- Use temporary files for log testing with --logfile='+'

- Change the symbol from '&' to '+' for using temporary files for
  logging with --logfile

- Enable skipped log tests again.

Bug: v8:10937, chromium:1129854, chromium:1130196
Change-Id: I607dc9a9ecc352e58525cdd21c1c93efebf0f09f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2421826
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70071}