[GN] Add a template which wraps generate_test_lists.py.

generate_test_lists.py is currently our solution for building and
running tests in an automated fashion. This CL adds a GN target,
cast_test_lists, which, when invoked, generates lists of tests to be
built and run.

Bug: b/23533616
BUG=516899

Review URL: https://codereview.chromium.org/1382713003

Cr-Commit-Position: refs/heads/master@{#352412}
6 files changed