[testbug] SliderTooltipNPETest fails on Linux

SliderTooltipNPETest fails consistently on Linux.

$ gradle -PFULL_TEST=true -PUSE_ROBOT=true cleanTest :systemTests:test --tests SliderTooltipNPETest

SliderTooltipNPETest > testSliderTooltipNPE FAILED
    java.lang.AssertionError: Timeout: Timeout waiting for tooltip to display
        at org.junit.Assert.fail(Assert.java:89)
        at org.junit.Assert.assertTrue(Assert.java:42)
        at test.util.Util.waitForLatch(Util.java:397)
        at test.robot.javafx.scene.SliderTooltipNPETest.dragSliderAfterTooltipDisplayed(SliderTooltipNPETest.java:94)
        at test.robot.javafx.scene.SliderTooltipNPETest.testSliderTooltipNPE(SliderTooltipNPETest.java:76)