Install python tests usus new libraries

waf install runs the version checks on the newly installed
python code, but it also uses the newly installed libraries
rather than the libraries the system would use by default.

Test case:
  on a Debian/Ubuntu system that needs a pth file
  install everything and get it working correctly
  rename ntpsec.pth to ntpsec.pth-x
  verify that ntpq doesn't work
  run waf install
  note that the python tests don't fail

There are 2 cases that this doesn't catch:
  one is a missing pth file
  the second is an old pth file pointing to old librarie