gnu: python-2.7: Include /lib/python.../test/support.

Don't remove the support directory from the /lib/python.../test/ directory, as
this is used from the test_support module.

  /gnu/store/...-python2-2.7.15/lib/python2.7/test/test_support.py:2: in <module>
      import test.support
  E   ImportError: No module named support

* gnu/packages/python.scm (python-2.7)[arguments]: Change the 'remove-tests
phase to not remove the support directory.