Implement test for WebCLDevice::getExtension
Issue #66

    Created test for all extensions supported by all devices available

    Reviewed by Afonso Costa

        * LayoutTests/webcl/resources/webcl-test-utils.js
        * LayoutTests/webcl/tests/webcl-test-device-extensions-expected.txt
        * LayoutTests/webcl/tests/webcl-test-device-extensions.html