tests: Add minimal constant buffer constructor

Most of the time we have everything needed to allocate and fill
a constant buffer. But occasionally want to create the constBufferObj
and fill in the details later.
diff --git a/tests/xglrenderframework.h b/tests/xglrenderframework.h
index 431ecd8..3149dee 100644
--- a/tests/xglrenderframework.h
+++ b/tests/xglrenderframework.h
@@ -125,6 +125,7 @@
 class XglConstantBufferObj
 {
 public:
+    XglConstantBufferObj(XglDevice *device);
     XglConstantBufferObj(XglDevice *device, int constantCount, int constantSize, const void* data);
     void SetMemoryState(XGL_CMD_BUFFER cmdBuffer, XGL_MEMORY_STATE newState);
     XGL_MEMORY_VIEW_ATTACH_INFO     m_constantBufferView;