Improve GN build test targets.

Add missing dependency on make_core_generated_private_script_for_testing.

Rename wtf:unittest_helpers to wtf:test_support for project consistency (I'm trying to standardize everything on "test_support").

Add blink:test_support target.

BUG=

Review URL: https://codereview.chromium.org/410083007

git-svn-id: svn://svn.chromium.org/blink/trunk@178788 bbb929c8-8fbe-4397-9dbb-9b2b20218538