LegacyHeadlessTest: add a test for headless SaveDialog instantiations

In the course of debugging

	http://fiji.sc/bugzilla/show_bug.cgi?id=630

it was conjectured that the SaveDialog cannot run in headless mode
without patching. This patch shows, however, that that is not true.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Legacy headless: support the ScaleDialog

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>