[TEST_BUG] Replace obsolete FXML tests that use Nashorn

Two of the FXML unit tests use the Nashorn JavaScript engine to test the FXML scripting capability. Nashorn has been deprecated since JDK 11 and has been removed in JDK 15.

When these tests are run with the default JDK 14, they pass, but produce a warning:

test.javafx.fxml.FXMLLoader_ScriptTest > testScriptHandler STANDARD_ERROR
    Warning: Nashorn engine is planned to be removed from a future JDK release

test.javafx.fxml.FXMLLoader_ScriptTest > testExternalScriptHandler STANDARD_ERROR
    Warning: Nashorn engine is planned to be removed from a future JDK release


When running with JDK 15-ea these two tests fail (as expected, since Nashorn is gone).

test.javafx.fxml.FXMLLoader_ScriptTest > testScriptHandler FAILED
    javafx.fxml.LoadException: Page language not specified.
    /C:/Users/kcr/javafx/jfx-kcr/jfx/rt/modules/javafx.fxml/build/resources/test/test/javafx/fxml/script_handler.fxml:35
        at javafx.fxml/javafx.fxml.FXMLLoader.constructLoadException(FXMLLoader.java:2625)
        at javafx.fxml/javafx.fxml.FXMLLoader$ScriptElement.processCharacters(FXMLLoader.java:1604)
        at javafx.fxml/javafx.fxml.FXMLLoader.processCharacters(FXMLLoader.java:2855)
        at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2566)
        at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2470)
        at javafx.fxml/javafx.fxml.FXMLLoader.load(FXMLLoader.java:2439)
        at test.javafx.fxml.FXMLLoader_ScriptTest.testScriptHandler(FXMLLoader_ScriptTest.java:134)

test.javafx.fxml.FXMLLoader_ScriptTest > testExternalScriptHandler FAILED
    javafx.fxml.LoadException: Unable to locate scripting engine for extension js.
    /C:/Users/kcr/javafx/jfx-kcr/jfx/rt/modules/javafx.fxml/build/resources/test/test/javafx/fxml/script_handler_external.fxml:31
        at javafx.fxml/javafx.fxml.FXMLLoader.constructLoadException(FXMLLoader.java:2625)
        at javafx.fxml/javafx.fxml.FXMLLoader$ScriptElement.processStartElement(FXMLLoader.java:1544)
        at javafx.fxml/javafx.fxml.FXMLLoader.processStartElement(FXMLLoader.java:2726)
        at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2556)
        at javafx.fxml/javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2470)
        at javafx.fxml/javafx.fxml.FXMLLoader.load(FXMLLoader.java:2439)
        at test.javafx.fxml.FXMLLoader_ScriptTest.testExternalScriptHandler(FXMLLoader_ScriptTest.java:147)


We should remove these two tests and replace them with a different (custom) javax.script implementation. See the tests added for JDK-8234959 for an example of how this might be done.