releasetools: Add FORCE_RUN_RELEASETOOLS.

This allows test runner to take care of the external tools setup, then
to start the test without a lunch'd environment. This is needed before
having supports like python_test_helper that packs dependents into
releasetools_test module.

Bug: 138791766
Test: Don't lunch any target. Use `FORCE_RUN_RELEASETOOLS=1 python -m
      unittest -v test_add_img_to_target_files` and check there's no
      skipped testcases.
Change-Id: Iafeaba54fc228d8e1fc5ff3c91f7ffebef18ad7f