Skip retries of PRE_ tests.
Skip retries of PRE_ tests.

Otherwise if a test fails in the PRE_ test case, run_test_cases will try to
retry it and will fail.

R=csharp@chromium.org
BUG=


Review URL: https://chromiumcodereview.appspot.com/12813008

git-svn-id: d5a9b8648c52d490875de6588c8ee7ca688f9ed1@189276 0039d316-1c4b-4281-b951-d872f2087c98

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