Tests for CurImagePlugin.py and DcxImagePlugin.py

Tests for CurImagePlugin.py
---

 * Test cursor image by Miss Mycroft.
  * 'Released under the Release to Public Domain license'.
  * http://www.rw-designer.com/cursor-detail/61757
 * Sanity test: open the image, check expected size and type.
 * Helps towards: Test coverage >= 80% #722.
 * flake8, typo.


---

Tests for DcxImagePlugin.py
---

 * 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.