Add "art/test" to the list of Core Library projects.

Allow modules under art/test to use `sdk_version: "none"`. This is so
that ART run-test module definitions can use the same settings as
Libcore tests:

  sdk_version: "none",
  system_modules: "core-all-system-modules",

Test: m nothing
Bug: 147814778
Change-Id: Ibb74d6a06a6609be511403c51ca7a02204bf606f