ENH: Do not setup the python tests if not asked

When building with python wrappings, the test files were downloaded and the tests were setup.
This is now done only if asked by the user, by setting BUILD_TESTING to ON

Change-Id: I81e711fb696083afc715d15ee0a3e1511ba91a41
Merge topic 'PythonTests'

4dd827dd ENH: Do not setup the python tests if not asked