Avoid that pesky MacOSX icon when running unit tests

Inspired by https://github.com/imagej/imagej/commit/d347d366.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'macosx-dock-icon'

This hides the annoying dock icon when running unit tests on MacOSX.

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