Run our HtmlUnit tests in module batch mode

This is a prerequisite to trapping window.onerror by default.

We might want to set that batch mode as the default going
forward (when using the HtmlUnit runStyle), but let's first
turn the switch for our own tests.

Change-Id: Iad2818b66658e24e54a661b5759cb99e4a0964fe

user/build.xml

@@ -43,7 +43,7 @@
   <property name="test.args.web.selenium"
             value='${test.args} -prod -out www -generateJsInteropExports -runStyle "Selenium:${gwt.hosts.web.selenium}"'/>
   <property name="test.args.web.htmlunit"
-            value='${test.args} -prod -out www -generateJsInteropExports'/>
+            value='${test.args} -prod -out www -generateJsInteropExports -batch module'/>
 
   <!--
     Whether I18NSuite should test e.g. Foo$InnerMsgs_fr.properties (if the