#1955 - Make unit test photos unique.

- Modified test::random_photo and test::random_photo_unsaved to, by default, uniquify the 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 replace_data_file_test and replace_data_file_test in Item_Model_Test to turn off uniquify (they need to know the exact filesize a priori).

Unit tests now take about ~10-20% longer to run, but we can be certain that all photos are unique.