Split out tests of private scripts from Internals to PrivateScriptTest

This is a clean-up CL of Blink-in-JS.
Instead of adding more tests about private scripts to Internals,
we should have a dedicated IDL file to handle those tests.

BUG=341031

Review URL: https://codereview.chromium.org/376553004

git-svn-id: svn://svn.chromium.org/blink/trunk@177629 bbb929c8-8fbe-4397-9dbb-9b2b20218538