Implement test for WebCLDevice::getExtension

Issue #66

    Created test for all extensions supported by device

    Reviewed by NOBODY (OOPS!).

    * LayoutTests/webcl/resources/webcl-test-utils.js
    * LayoutTests/webcl/tests/webcl-test-device-extensions-expected.txt
    * LayoutTests/webcl/tests/webcl-test-device-extensions.html