releasetools: Add unittests for GetCareMap() and AddCareMapTxtForAbOta().

With test_utils.construct_sparse_image(), creating valid sparse image is
no longer a blocker for testing these functions.

Test: python -m unittest test_add_img_to_target_files
Change-Id: Iff1f15837cc744bad52e2120f9a9ad94d7db50d5