TT#121955 py-test: define cache_dir to avoid PytestCacheWarning

> /usr/lib/python3/dist-packages/_pytest/stepwise.py:122
>   /usr/lib/python3/dist-packages/_pytest/stepwise.py:122: PytestCacheWarning: could not create cache path /code/.pytest_cache/v/cache/stepwise
>     self.config.cache.set("cache/stepwise", [])
>
> /usr/lib/python3/dist-packages/_pytest/cacheprovider.py:396
>   /usr/lib/python3/dist-packages/_pytest/cacheprovider.py:396: PytestCacheWarning: could not create cache path /code/.pytest_cache/v/cache/nodeids
>     config.cache.set("cache/nodeids", sorted(self.cached_nodeids))
>
> /usr/lib/python3/dist-packages/pytest_pep8.py:44
>   /usr/lib/python3/dist-packages/pytest_pep8.py:44: PytestCacheWarning: could not create cache path /code/.pytest_cache/v/pep8/mtimes
>     config.cache.set(HISTKEY, config._pep8mtimes)

Change-Id: Ia5d6ebd823d1a51967a0b6d0a64f534a1fc1f577

tox.ini

@@ -11,6 +11,7 @@ filterwarnings =
 #log_cli = true
 #log_cli_level = DEBUG
 pep8ignore = W503
+cache_dir = ${RESULTS}/.__pycache__
 
 [pep8]
 ignore = W503