Test improvements

[akx]

This PR (required for #313):

• refactors tests to split them into more files so they need to be annotated less when PIL is not available
• rewrites the unittest-style assertions to regular assertions (since pytest is being used to run things)
• rewrites the unittest-style tests to pytest-style functions (and pytest-style fixtures)
• parametrizes tests to get rid of redundant code

[akx]

@maribedran @bartTC I can rebase this cleanly - let me know on what...

[maribedran]

Thanks, @akx ! I already merged you changes to #370 .