tests: fix depth image preparation

Should prepare m_depthStencilImage, not m_renderTarget->image().
diff --git a/tests/render_tests.cpp b/tests/render_tests.cpp
index 5bbcc1e..fe32a5b 100644
--- a/tests/render_tests.cpp
+++ b/tests/render_tests.cpp
@@ -448,7 +448,7 @@
 
             // prepare depth buffer for rendering
             XGL_IMAGE_STATE_TRANSITION transitionToRender = {};
-            transitionToRender.image = m_renderTarget->image();
+            transitionToRender.image = m_depthStencilImage;
             transitionToRender.oldState = XGL_IMAGE_STATE_CLEAR;
             transitionToRender.newState = m_depthStencilBinding.depthState;
             transitionToRender.subresourceRange = dsRange;