tests/org.eclipse.core.tests.runtime/Plugin_Testing/softPrereq.2/tests.a/plugin.xml

<?xml version="1.0" encoding="UTF-8"?>
<plugin
  name="IBM Tooling Platform Core Tests"
  id="tests.a"
  version="1.2"
  vendor-name="IBM">

  <requires>
     <import plugin="tests.b" optional="true"/>
  </requires>

</plugin>