Tests: Be verbose and fail fast

Two things:

Be verbose
* Rather than printing just a dot, show the test name so we can see which tests pass and are skipped.
* To do this, override `__str__` in `PillowTestCase to` give nicer output (`TestCffiGetPixel.test_signedness` instead of `test_signedness (test_cffi.TestCffiGetPixel)`.
* Also use unique class names to match filenames (fixes copy-paste errors).
* Merge some similar tests (e.g. test_imagefilter.py and test_image_filter.py) that don't need to be in separate files.

Fail fast
* Stop running tests. after the first error or failure. This means we get feedback sooner from Travis rather than having to wait for the whole test batch to run, which can save considerable time.