Change default Parameters for screenshot testing to be more relaxed

[TatuLund]

The default Parameter's in screen shot testing are strict. This means that tests will be flaky.

For example I am using in my add-on testing the following settings

        // We do screenshot testing, adjust settings to ensure less flakiness
        Parameters.setScreenshotComparisonTolerance(0.05);
        Parameters.setScreenshotComparisonCursorDetection(true);
        testBench().resizeViewPortTo(800, 600);
        Parameters.setMaxScreenshotRetries(3);
        Parameters.setScreenshotRetryDelay(1000);

The default view port of app testing should be larger than 800 x 600 I use for add-ons. Point here is that we should ensure some fixed default size, since otherwise the view port size can wary.

More over I sometimes run tests also in Development mode, when dev mode gizmo needs to be hidden

        // Hide dev mode gizmo, it would interfere screenshot tests
        try {
            $("vaadin-dev-tools").first().setProperty("hidden", true);
        } catch (NotFoundException e) {

        }

Screenshot testing would be more approachable if the defaults would be something like this. Now developers need to consume time on trial and error to find these.