[TEST BUG] Exceptions, warnings when running TabPaneDragPolicyTest

The TabPaneDragPolicyTest has two problems that are causing exceptions and warning messages to be printed in the test. This is not a product bug, but are due to problems with the test itself.

To reproduce run the test as follows:

gradle --info -PFULL_TEST=true -PUSE_ROBOT=true :systemTests:cleanTest :systemTests:test --tests test.robot.javafx.scene.TabPaneDragPolicyTest

Several exceptions will be written to the "Standard error" log in the test report of the form:

    Exception in thread "Test worker" java.lang.IllegalStateException: Not on FX application thread; currentThread = Test worker
        at javafx.graphics/com.sun.javafx.tk.Toolkit.checkFxUserThread(Toolkit.java:291)
        at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.checkFxUserThread(QuantumToolkit.java:423)
        at javafx.graphics/javafx.scene.Parent$3.onProposedChange(Parent.java:478)
        at javafx.base/com.sun.javafx.collections.VetoableListDecorator.setAll(VetoableListDecorator.java:113)
        ...
        at javafx.graphics/javafx.scene.Node.applyCss(Node.java:9599)
        at javafx.controls/javafx.scene.control.skin.TabPaneSkin.updateTabPosition(TabPaneSkin.java:666)
        at javafx.controls/javafx.scene.control.skin.TabPaneSkin.lambda$new$1(TabPaneSkin.java:203)
        at javafx.controls/com.sun.javafx.scene.control.LambdaMultiplePropertyChangeListenerHandler.lambda$new$1(LambdaMultiplePropertyChangeListenerHandler.java:49)
        at javafx.base/javafx.beans.value.WeakChangeListener.changed(WeakChangeListener.java:86)
        at javafx.base/com.sun.javafx.binding.ExpressionHelper$Generic.fireValueChangedEvent(ExpressionHelper.java:360)
        at javafx.base/com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:80)
        at javafx.base/javafx.beans.property.ObjectPropertyBase.fireValueChangedEvent(ObjectPropertyBase.java:106)
        at javafx.base/javafx.beans.property.ObjectPropertyBase.markInvalid(ObjectPropertyBase.java:113)
        at javafx.base/javafx.beans.property.ObjectPropertyBase.set(ObjectPropertyBase.java:147)
        at javafx.controls/javafx.scene.control.TabPane.setSide(TabPane.java:202)
        at test.robot.javafx.scene.TabPaneDragPolicyTest.testReorderBottom(TabPaneDragPolicyTest.java:155)

Additionally, 4 warnings will show up in the test report in the "Standard output" log as follows:

    Timeout waiting ChangeListener to get called.

Note that the exceptions will also be printed to the console (the warnings won't because they were mistakenly written to System.out rather than System.err).