[modular][testing][sdk] Modular Testing SDK

* //sdk:modular_testing is an SDK that provides testing utilities for
  writing app & integration tests that run under modular. This CL
  bootstraps this SDK with just the runtime necessary to use the
  fuchsia.modular.testing.TestHarness.

* sdk_image.gni now contains the modular_test_harness fuchsia package,
  which is required by //sdk:modular_testing

MF-328

Test: The c++ gtest fixture is tested by test_harness_fixture_test
Change-Id: I3e4944ac25ecf4cf14960d6386ce64e8a2a935d6