Restructured oes-element-index-uint.html to act as a regression test for a bug in ANGLE which has since been fixed.

The component tests under conformance/buffers/ were unaffected by this bug, so not modifying them.

Bug: https://github.com/KhronosGroup/WebGL/issues/328
Merge pull request #332 from kenrussell/dynamic-draw

Restructured oes-element-index-uint.html to act as a regression test for...