IDL compiler: turn on Python testing by default in run-bindings-tests

Now that we're about to switch compilers, having run-bindings-tests
test Python too helps prevent any last-minute problems from in-flight
CLs landing while we're switching.

BUG=239771
R=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/178073005

git-svn-id: svn://svn.chromium.org/blink/trunk@167838 bbb929c8-8fbe-4397-9dbb-9b2b20218538