java.lang.NoClassDefFoundError: org/hamcrest/SelfDescribing despite having hamcrest on classpath

Question

When attempting to run tests with JUnit 5 via the Ant junitlauncher task, I receive the following message:

[junitlauncher] WARNING: TestEngine with ID 'junit-jupiter' failed to execute tests [junitlauncher] java.lang.NoClassDefFoundError: org/hamcrest/SelfDescribing

How do I fix this so that my tests can run?

The answers to similar questions (e.g. this one) mainly say to make sure that hamcrest is on the classpath. I've verified using the print-test-classpath target (see build file below for details) that the classpath I'm using to run my tests includes hamcrest-2.1.jar. Furthermore, the tests compile without any issue.

Here are the sections of my build.xml I think are relevant:

<path id="lib-only-class.path">
  <fileset dir="${lib.dir}">
    <include name="**/*.jar" />
  </fileset>
</path>

<path id="class.path">
  <path refid="lib-only-class.path"/>
  <pathelement location="${classes.dir}"/>
</path>

<path id="test-class.path">
  <path refid="class.path"/>
  <pathelement location="${test-classes.dir}"/>
</path>

<target name="compile-tests" depends="compile">
  <mkdir dir="${test-classes.dir}"/>
  <javac srcdir="${test.dir}" destdir="${test-classes.dir}"
         includeantruntime="no" classpathref="class.path"/>
</target>

<target name="test" depends="compile-tests">
  <junitlauncher printsummary="yes">
    <classpath refid="test-class.path"/>
    <testclasses>
      <fileset dir="${test-classes.dir}"/>
    </testclasses>
  </junitlauncher>
</target>

<target name="print-test-classpath">
  <pathconvert property="classpathInName" refid="test-class.path"/> 
  <echo>Classpath is ${classpathInName}</echo>
</target>