#1955 - Make unit test photos unique.
- Added test::random_unique_photo and test::random_unique_photo_unsaved to uniquify test photos.
- Uniquified the black dot of test.jpg by coloring it with the six-digit hex code already used to name the random photos (e.g. "name_a48801.jpg").
- Modified four tests in Item_Model_Test that check photo file contents to use new functions to guarantee uniqueness.