libxml2: Make ptest run the Python tests if Python support is enabled

libxml2: Make ptest run the Python tests if Python support is enabled

Since we go through the trouble of copying the Python tests, we may as
well actually run them...

This also avoids the following QA issue:

  ERROR: libxml2-2.9.4-r0 do_package_qa: QA Issue:
  /usr/lib/libxml2/ptest/python/tests/push.py contained in package
  libxml2-ptest requires /usr/bin/python, but no providers found in
  RDEPENDS_libxml2-ptest? [file-rdeps]

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>