Skip scriptdef-test.xml antunit tests for JDK >=15 due to unavailability of script engine for "javascript" language in JRE

src/tests/antunit/taskdefs/optional/script/scriptdef-test.xml

@@ -26,13 +26,17 @@
   </description>
 
   <condition property="prereqs-ok">
-    <or>
-      <and>
-        <available classname="org.apache.bsf.BSFManager" />
-        <available classname="org.apache.bsf.engines.javascript.JavaScriptEngine" />
-      </and>
-      <available classname="javax.script.ScriptEngineManager" />
-    </or>
+    <and>
+      <!-- Starting Java 15, there's no "javascript" script engine (not even nashorn) bundled in JRE -->
+      <javaversion atmost="14"/>
+      <or>
+        <and>
+          <available classname="org.apache.bsf.BSFManager" />
+          <available classname="org.apache.bsf.engines.javascript.JavaScriptEngine" />
+        </and>
+        <available classname="javax.script.ScriptEngineManager" />
+      </or>
+    </and>
   </condition>
 
   <!-- auto doesn't verify the language is supported and selects BSF