function get_harness_url()
    {
        var script_url = get_script_url();

        // Exclude the 'testharness.js' file from the returned path, but '+ 1' to include the trailing slash.
        return script_url ? script_url.slice(0, script_url.lastIndexOf('/') + 1) : undefined;
    }
Returns the URL path at which the files for testharness.js are assumed to reside (e.g., '/resources/').
        The path is derived from inspecting the 'src' of the <script> tag that included 'testharness.js'.