au_test_harness: race condition when creating directories

Avoid race condition in creating download folder when running VMTest in
parallel.

BUG=None
TEST=Running ctest manually.

Change-Id: I4d233777ca297d5a86359ec5c4e1b90979e2859d
Reviewed-on: https://chromium-review.googlesource.com/792091
Commit-Ready: Po-Hsien Wang <pwang@chromium.org>
Tested-by: Po-Hsien Wang <pwang@chromium.org>
Reviewed-by: Ilja H. Friedel <ihf@chromium.org>
1 file changed