releasetools: Add unittest for common.GetSparseImage().

Add construct_sparse_image() to test_utils.py, which is a util function
to create sparse images. The new tests also partially cover the recent
changes that add 'incomplete' and 'uses_shard_blocks' tags.

Test: python -m unittest test_common
Change-Id: Ia15f5c4ad12423691216ebbad2c28f95c8427d7e