The python tests currently use the default desktop-dependent vclplug,
but should use svp like every other test.

Apparently there's no way to pass command line parameters through
python, so set SAL_USE_VCLPLUGIN=svp.

Change-Id: Idb9afd87a1721c5188aeecd9b1f83521f6431757
(cherry picked from commit a9833ee9)