I've posted this answer to this question before realizing that it was a duplicate of yours.
Here is my junit tag that does produce the exception trace.
<junit
showoutput="yes"
errorProperty="test.failed"
failureProperty="test.failed"
haltOnFailure="${test.halt-on-failure}"
fork="yes"
forkmode="${junit.forkmode}"
>
<classpath>
<pathelement location="${classes.dir}"/>
<pathelement location="${classes-under-test.classes.dir}" />
</classpath>
<!-- #Formatters for capture and display -->
<formatter
type="brief"
usefile="false"
/>
<formatter type="brief" />
<formatter
type="xml"
if="test.generate.xml.output"
/>
<!-- #Test case isolation technique -->
<test
name="${testcase}"
if="testcase"
/>
<batchtest
todir="${test.data.dir}"
unless="testcase"
>
<fileset dir="${classes.dir}">
<include name="**/Test*.class" />
<exclude name="**/Test*$*.class" />
</fileset>
</batchtest>
</junit>
I think the one nested element that will do it for you is
<formatter
type="brief"
usefile="false"
/>