Fix run_isolated_smoke_test to work when /tmp is on a separate partition.
Fix run_isolated_smoke_test to work when /tmp is on a separate partition.

This was broken since r211201 when the current working directory is different
from /tmp, it would throw an exception. This is necessary for performance
concerns.

R=vadimsh@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/23380003

git-svn-id: d5a9b8648c52d490875de6588c8ee7ca688f9ed1@219566 0039d316-1c4b-4281-b951-d872f2087c98

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py