UI-Tests are broken under MacOS

[kaklakariada]

When running UI-Tests in non-headless mode under macOS with the following command:

./gradlew jfxui:uiTest -i --tests AutomaticInterruptionTest  -PuiTestsHeadless=false

the tests fail with the following error:

AutomaticInterruptionTest > stopWorkingForTodayButtonDoesNotAddInterruptionDoesNotUpdateEnd() FAILED
    java.lang.AssertionError: Expected: Window is not showing
         but: javafx.scene.control.HeavyweightDialog$1@38fb76a5
        at org.testfx.assertions.impl.Adapter$1.matches(Adapter.java:55)
        at org.assertj.core.internal.Conditions.assertIs(Conditions.java:61)
        at org.assertj.core.api.AbstractAssert.is(AbstractAssert.java:433)
        at org.testfx.assertions.api.AbstractWindowAssert.isNotShowing(AbstractWindowAssert.java:52)
        at org.itsallcode.whiterabbit.jfxui.testutil.model.AutomaticInterruptionDialog.clickStopWorkForToday(AutomaticInterruptionDialog.java:51)
        at org.itsallcode.whiterabbit.jfxui.AutomaticInterruptionTest.lambda$stopWorkingForTodayButtonDoesNotAddInterruptionDoesNotUpdateEnd$2(AutomaticInterruptionTest.java:59)
        at org.itsallcode.whiterabbit.jfxui.AutomaticInterruptionTest.simulateInterruption(AutomaticInterruptionTest.java:78)
        at org.itsallcode.whiterabbit.jfxui.AutomaticInterruptionTest.stopWorkingForTodayButtonDoesNotAddInterruptionDoesNotUpdateEnd(AutomaticInterruptionTest.java:59)

The same tests run successfully under Linux.