It looks like the build file wasn't updated when JBoss moved to ant-1.6.x.
In order to get the testsuite to compile, I had to change the following:

<!-- Classpath to build and run the tests -->
<path id="tests.classpath">
<pathelement path="${classpath}"/>
<pathelement path="${local.classpath}"/>
<pathelement path="${project.tools}/lib/ant.jar"/>

• <pathelement path="${project.tools}/lib/optional.jar"/>
  + <pathelement path="${project.tools}/lib/ant-junit.jar"/>
  <path refid="jboss.jmx.classpath"/>
  <path refid="thirdparty.classpath"/>
  </path>