Set SAL_USE_VCLPLUGIN=svp on all platforms to define headless mode

... as used now in ODatabaseSource::buildLowLevelConnection; so not
having it on Windows/macOS makes it hang waiting for dialog (e.g., in
fdo84315).

Change-Id: Ia63921ae398eddcd1be7d23e75c85979109a6615
Reviewed-on: https://gerrit.libreoffice.org/54068Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>
Tested-by: Mike Kaganski <mike.kaganski@collabora.com>

solenv/gbuild/PythonTest.mk

@@ -52,9 +52,9 @@ else
 		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 \
-		$(if $(filter-out MACOSX WNT,$(OS_FOR_BUILD)),$(if $(ENABLE_HEADLESS),, \
+		$(if $(ENABLE_HEADLESS),, \
 			SAL_USE_VCLPLUGIN=svp \
-		)) \
+		) \
 		$(gb_CppunitTest_GDBTRACE) $(gb_CppunitTest_VALGRINDTOOL) \
 			$(gb_PythonTest_COMMAND) \
 			$(if $(PYTHON_TEST_NAME),$(PYTHON_TEST_NAME),$(MODULES)) \