Don't use PLY cache files in Tools/Scripts/run-bindings-tests

We don't need the caching, and disabling it avoids cache coherency issues
as well as leaving untracked files in the tree.

BUG=426339

Review URL: https://codereview.chromium.org/646283004

git-svn-id: svn://svn.chromium.org/blink/trunk@184249 bbb929c8-8fbe-4397-9dbb-9b2b20218538