Fix GL entry point loading in robustness tests
Fix GL entry point loading in robustness tests

Tests were not loading robustness-related entry points if implementation
supported only GL_EXT_robustness and thus had EXT-suffixed entry points.

Bug: 36564789
Bug: 34887315
Change-Id: I70e610dd3083e7c09050e24ef829c71f950d82a1

googlestadia/VK-GL-CTS,KhronosGroup/Vulkan-CTS,chadversary/deqp,chadversary/deqp,KhronosGroup/Vulkan-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,chadversary/deqp,KhronosGroup/VK-GL-CTS,chadversary/deqp,KhronosGroup/Vulkan-CTS,googlestadia/VK-GL-CTS,googlestadia/VK-GL-CTS,chadversary/deqp,chadversary/deqp,KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,googlestadia/VK-GL-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,googlestadia/VK-GL-CTS,googlestadia/VK-GL-CTS,KhronosGroup/VK-GL-CTS