Remove in-application test harness

[1ec5]

The in-application test harness, accessible only in debug builds via the Test button in the options dialog, no longer displays a file chooser and uses obsolete interfaces (#189). As of 87e5111849540bc0d6a2f2dd2c45afa9e4adb2f5, there’s a newer test harness that embeds AVIM inside a webpage for much easier testing, so there’s little point to keeping the older test harness around.