Tests: fix cffi caching for DSP files.

Also fix cffi caching in the test suite.
Tests: clean up cffi cache in tearDownModule().

Move the cffi cache cleanup into the module-level tear down function
tearDownModule().  In combination with the cffi cache fix, this speeds up the
test suite quite noticeably.
Delete some silly whitespace.
PythonDSP: replace __del__ with an ffi.gc hook.

Replace the __del__ method with an ffi.gc hook that calls deletemydsp() once
self.__dsp is garbage collected.