gbuild: PythonTest: run tests with --headless

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)

solenv/gbuild/PythonTest.mk

@@ -52,7 +52,11 @@ $(call gb_PythonTest_get_target,%) :| $(gb_PythonTest_DEPS)
 		UserInstallation=$(call gb_Helper_make_url,$(dir $(call gb_PythonTest_get_target,$*))user) \
 		TestUserDir="$(call gb_Helper_make_url,$(dir $(call gb_PythonTest_get_target,$*)))" \
 		PYTHONDONTWRITEBYTECODE=1 \
-		$(gb_CppunitTest_GDBTRACE) $(gb_CppunitTest_VALGRINDTOOL) $(gb_PythonTest_COMMAND) \
+		$(if $(filter-out MACOSX WNT,$(OS_FOR_BUILD)),$(if $(ENABLE_HEADLESS),, \
+			SAL_USE_VCLPLUGIN=svp \
+		)) \
+		$(gb_CppunitTest_GDBTRACE) $(gb_CppunitTest_VALGRINDTOOL) \
+			$(gb_PythonTest_COMMAND) \
 			$(MODULES) \
 		$(if $(gb_CppunitTest__interactive),, \
 			> $@.log 2>&1 \