Implement Layout Test: One Context for each Device

        Issue #44

        Reviewed by Alexandre Rocha, Afonso Costa.

        * webcl/resources/webcl-proxy.js:
        * webcl/resources/webcl-test-utils.js:
        * webcl/tests/webcl-test-devices-multiple-contexts-expected.txt: Added.
        * webcl/tests/webcl-test-devices-multiple-contexts.html: Added.