Make sure oes-element-index-uint test queries for attribute locations before binding

I recently enabled symbol name mangling in WebKit and noticed that this particular test was assuming that the attribute indices match the order they appear in the shader. There are many tests that do the same, but this was the only one I hit that actually changed order (I might hit more on other hardware).

In general though I'm not sure whether I should do something in WebKit to avoid this. I expect there is a lot of existing content that does not check attribute locations before binding.