Prevent gradle test on macOS continuously grabbing focus

When gradle runs tests on macOS, it is continuously launching new JVMs during the tests. These java invocations create a new Java icon on the Dock, and when they launch they grab window/mouse focus, making it difficult to focus on another task.

Setting system property
apple.awt.UIElement=true
at JVM launch prevents the icon from appearing, and if the focus is elsewhere it doesn't continuously get shifted.

This is similar to java.awt.headless=true but that property also does other things in Jalview.