Cache collisions in test suite

(The CI tests all get run with LOOPY_NO_CACHE=1 so these are not reported in CI).

I am using Python 3.6 if it matters.

test/test_loopy.py::test_constant_array_args[ctx_factory=<context factory for <pyopencl.Device 'pthread-Intel(R) Core(TM) i7-5650U CPU @ 2.20GHz' on 'Portable Computing Language' at 0x1c4d920>>]
  /home/matt/src/pytools/pytools/persistent_dict.py:413: UserWarning: loopy-preprocess-cache-v2-v66-islpy2016.2.2: key collision in cache at '/home/matt/.cache/pytools/pdict-v2-loopy-preprocess-cache-v2-v66-islpy2016.2.2-py3.6.0.final.0' -- these are sufficiently unlikely that they're often indicative of a broken implementation of equality comparison
    % (self.identifier, self.container_dir))
  /home/matt/src/pytools/pytools/persistent_dict.py:413: UserWarning: loopy-typed-and-scheduled-cache-v1-v66-islpy2016.2.2: key collision in cache at '/home/matt/.cache/pytools/pdict-v2-loopy-typed-and-scheduled-cache-v1-v66-islpy2016.2.2-py3.6.0.final.0' -- these are sufficiently unlikely that they're often indicative of a broken implementation of equality comparison
    % (self.identifier, self.container_dir))
  /home/matt/src/pytools/pytools/persistent_dict.py:413: UserWarning: loopy-code-gen-cache-v3-v66-islpy2016.2.2: key collision in cache at '/home/matt/.cache/pytools/pdict-v2-loopy-code-gen-cache-v3-v66-islpy2016.2.2-py3.6.0.final.0' -- these are sufficiently unlikely that they're often indicative of a broken implementation of equality comparison
    % (self.identifier, self.container_dir))

test/test_loopy.py::test_header_extract
  /home/matt/src/pytools/pytools/persistent_dict.py:413: UserWarning: loopy-preprocess-cache-v2-v66-islpy2016.2.2: key collision in cache at '/home/matt/.cache/pytools/pdict-v2-loopy-preprocess-cache-v2-v66-islpy2016.2.2-py3.6.0.final.0' -- these are sufficiently unlikely that they're often indicative of a broken implementation of equality comparison
    % (self.identifier, self.container_dir))
  /home/matt/src/pytools/pytools/persistent_dict.py:413: UserWarning: loopy-schedule-cache-v4-v66-islpy2016.2.2: key collision in cache at '/home/matt/.cache/pytools/pdict-v2-loopy-schedule-cache-v4-v66-islpy2016.2.2-py3.6.0.final.0' -- these are sufficiently unlikely that they're often indicative of a broken implementation of equality comparison
    % (self.identifier, self.container_dir))
  /home/matt/src/pytools/pytools/persistent_dict.py:413: UserWarning: loopy-code-gen-cache-v3-v66-islpy2016.2.2: key collision in cache at '/home/matt/.cache/pytools/pdict-v2-loopy-code-gen-cache-v3-v66-islpy2016.2.2-py3.6.0.final.0' -- these are sufficiently unlikely that they're often indicative of a broken implementation of equality comparison
    % (self.identifier, self.container_dir))

test/test_loopy.py::test_integer_reduction[ctx_factory=<context factory for <pyopencl.Device 'pthread-Intel(R) Core(TM) i7-5650U CPU @ 2.20GHz' on 'Portable Computing Language' at 0x1c4d920>>]
  /home/matt/src/pytools/pytools/persistent_dict.py:413: UserWarning: loopy-typed-and-scheduled-cache-v1-v66-islpy2016.2.2: key collision in cache at '/home/matt/.cache/pytools/pdict-v2-loopy-typed-and-scheduled-cache-v1-v66-islpy2016.2.2-py3.6.0.final.0' -- these are sufficiently unlikely that they're often indicative of a broken implementation of equality comparison
    % (self.identifier, self.container_dir))
  /home/matt/src/pytools/pytools/persistent_dict.py:413: UserWarning: loopy-preprocess-cache-v2-v66-islpy2016.2.2: key collision in cache at '/home/matt/.cache/pytools/pdict-v2-loopy-preprocess-cache-v2-v66-islpy2016.2.2-py3.6.0.final.0' -- these are sufficiently unlikely that they're often indicative of a broken implementation of equality comparison
    % (self.identifier, self.container_dir))