Unit tests for `isSameImage()`

[hrj]

Since image comparison is the keystone of the testing process, we need to test the isSameImage implementation thoroughly.

Some ideas:

• Need to create sets of similar images (some with small differences, some with larger diffs).
• Test with and without thresholds
• Test with same size and different size images
• Test missing images

Lesser priority tests:

• Test corrupted images
• Test hue variations
• Test transparency

[hrj]

@atiqsayyed Thanks for PR #18

Do you want to continue adding more tests for isSameImage or would like to pick up something else?

BTW, I have committed some minor changes on master (avoids creating driver for image tests).

[atiqsayyed]

@hrj it depends on what else is on priority...
If we're fine with having the basic tests in place then I can pick something else which may be of higher priority

[hrj]

@atiqsayyed working on image comparison tests is better for now because it is independent of other work, so there will be minimal interference.