tests: Find supported depth format instead of assuming

that VK_FORMAT_D24_UNORM_S8_UINT is always supported

Change-Id: I8fa4b0f163e0208ffee8c18e4bbdeaf7008f8730
diff --git a/tests/vkrenderframework.h b/tests/vkrenderframework.h
index f4b56ec..b36d254 100644
--- a/tests/vkrenderframework.h
+++ b/tests/vkrenderframework.h
@@ -437,4 +437,5 @@
     vector<VkPipelineColorBlendAttachmentState> m_colorAttachments;
     int m_vertexBufferCount;
 };
+VkFormat find_depth_stencil_format(VkDeviceObj *device);
 #endif  // VKRENDERFRAMEWORK_H