The junit tag take in a formatter tag to define the output, and classpath so it can find the classes, and a fileset within a batchtest tag to say where the test classes are (** means recursive).

<target name="junit" depends="compile">
    <junit printsummary="on" fork="true" haltonfailure="yes">
       <formatter type="plain" />
       <classpath refid="junit.class.path" />
       <batchtest todir="${test.report.dir}">
         <fileset dir="${src.dir}">
          <include name="**/*Test.java" />
         </fileset>
       </batchtest>
    </junit>
</target>   

The printsummary, fork and haltonfailure options does as you would expect. Batchtest uses the report dir to place the reports. You can go there to see the test output.