The thesaurus.xsd file that is part of the Full Text Test Suite begins with:

<xs:schema targetNamespace="http://www.w3.org/2007/xqftts/thesaurus"
   elementFormDefault="qualified"
   xmlns="http://www.w3.org/xqftts/thesaurus"
   xmlns:xs="http://www.w3.org/2001/XMLSchema">

Shouldn't line 3 instead be:

   xmlns="http://www.w3.org/2007/xqftts/thesaurus"

?  (The "2007" is missing.)  Without the change, the following query fails because the document doesn't validate:

import schema "http://www.w3.org/2007/xqftts/thesaurus"
  at "file:///path/to/w3c_full_text_testsuite/TestSources/thesaurus.xsd";
    
validate { doc( "file:///path/to/w3c_full_text_testsuite/TestSources/usability2.xml" ) }

With the change, it validates just fine.