This test references variable $b, which is not defined.  The test should expect XPST0008.  I'm guessing the test was meant be:

        declare variable $a := $x;
        declare variable $x external := $a + $x;
        $x


   <test-case name="extvardef-011">
     <description>Cycles are forbidden. </description>
      <created by="David Graf" on="2011-12-02"/>
      <dependency type="spec" value="XQ30+"/>
      <test><![CDATA[
        declare variable $a := $x;
        declare variable $x external := $a + $b;
        $x
      ]]></test>
      <result>
         <error code="XQST0054"/>
      </result>
   </test-case>