Support require_root in auto-gen test configs

require_root is added to allow auto-generated test config to include
RootTargetPreparer so the test runs with root permission.

Bug: 134509111
Test: add "require_root: true" to init_benchmarks and libpower_test
  build the modules, confirm the extra target preparer is added in the
  test configs.

Change-Id: Ia07503e338935d6aa92560e7cf7b18d2a4c51243