Tests for DcxImagePlugin.py

 * Branch accidentally includes #804. Let me know if I should redo this PR from a clean branch.

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 * Test image created with ImageMagick: `convert lena.ppm lena.dcx`
 * Sanity test: open the image, check expected size and type, and that it's the same image as the original.
 * Test `tell()`, and test `seek()`ing too far.
 * Helps towards: Test coverage >= 80% #722.
 * flake8.