Add spurious and missing test cases

Author: atorralba

java/ql/test/library-tests/xml/test.xml

@@ -1,4 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <document>
     <element attribute="value">Text</element> <!-- $ hasXmlResult -->
+    <element attribute="value">Text</element> <!-- Missing -->
+    <element>Text</element> <!-- $ hasXmlResult --> <!-- Spurious -->
 </document>