Widget tests fail when closing/reopening notebooks with saved widget output

[DonJayamanne]

Problem with this test is simple, when we open a notebook with an existing widget output,
then the widget renderer loads first, not the test script, even though the
test scripts have been defined in the controller script dependencies as the first script in list of scripts to be loaded.
Hence the widget renderer cannot find the test script and we cannot communicate with the widget renderer.

[DonJayamanne]

Can be fixed using IPyWidgets 8 (new protocol methods in kernel)

[DonJayamanne]

Closing in favour of #14856