The test fails intermittently on Mac, which is reported as JDK-8213136.
Looking at the behavior it seems like a product issue which can be only observed on Mac. The test should be disabled until we fix JDK-8213136.

Also, Below exception observed on Mac and Windows with FIXED drag policy tests, (have not verified on Linux).
As these changes are minor and specific to this test, It should be acceptable to push the fix under one bug.

test.robot.javafx.scene.TabPaneDragPolicyTest > testFixedLeft STANDARD_ERROR
    Exception in thread "Test worker" java.lang.IllegalStateException: This operation is permitted on the event thread only; currentThread = Test worker
        at javafx.graphics/com.sun.glass.ui.Application.checkEventThread(Application.java:441)
        at javafx.graphics/com.sun.glass.ui.View.getNativeView(View.java:450)
        at javafx.graphics/com.sun.glass.ui.win.WinAccessible.get_HostRawElementProvider(WinAccessible.java:687)
        at javafx.graphics/com.sun.glass.ui.win.WinAccessible.UiaRaiseAutomationEvent(Native Method)
        at javafx.graphics/com.sun.glass.ui.win.WinAccessible.sendNotification(WinAccessible.java:290)
        at javafx.graphics/javafx.scene.Node.notifyAccessibleAttributeChanged(Node.java:10032)
        at javafx.controls/javafx.scene.control.TabPane$TabPaneSelectionModel.select(TabPane.java:745)
        at javafx.controls/javafx.scene.control.TabPane$TabPaneSelectionModel.select(TabPane.java:754)
        at javafx.controls/javafx.scene.control.TabPane$TabPaneSelectionModel.select(TabPane.java:673)
        at test.robot.javafx.scene.TabPaneDragPolicyTest.testReorder(TabPaneDragPolicyTest.java:293)
        at test.robot.javafx.scene.TabPaneDragPolicyTest.testFixedLeft(TabPaneDragPolicyTest.java:225)