Now that it is not necessary to put junit.jar into $CLASSPATH to run tests, need to make sure -Dtestcase=... works too.

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@944382 13f79535-47bb-0310-9956-ffa450edef68

build.xml

@@ -1586,7 +1586,7 @@ see ${build.junit.reports} / ${antunit.reports}
     </condition>
 
     <condition property="junit.testcase" value="${testcase}">
-      <available classname="${testcase}" classpathref="tests-runtime-classpath" />
+      <available classname="${testcase}" classpathref="tests-runtime-classpath" ignoresystemclasses="true"/>
     </condition>
 
     <fail>Cannot locate test ${testcase}