More tests for Image.py

 * Add more tests for `Image`'s previously uncovered functions.
 * Helps towards: Test coverage >= 80% #722.

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About `test_alpha_composite()`:

`Image.alpha_composite()` was added in https://github.com/python-pillow/Pillow/pull/23.

The alpha composite test is based on this test mentioned in the original PR, but the test doesn't appear to have been committed: https://github.com/python-pillow/Pillow/pull/23#issuecomment-11081185.

At first, the only logical changes to the test was sorting the colour lists before comparison. However it failed. 

The images -- `im1`, `im2` and `im3` saved to png -- look valid with a visual check. There were subsequent changes to alpha compositing in https://github.com/python-pillow/Pillow/pull/29, https://github.com/python-pillow/Pillow/pull/156 and https://github.com/python-pillow/Pillow/pull/159, so I've modified the expected values to match the current implementation.