[test] enforce defragmentation of XML resources

By setting the allowed-fragmentation attribute value to -1 any
modification will trigger the defragmentation of the structural(?) index (AKA BTree).

This setting triggers the NPE described in eXist-db#5273 on the very first call to

update (replace|insert)

Unfortunately, it also triggers a totally different class of NPEs in
XQueryTrigger and other subsystems of the database.

exist-core/src/test/resources-filtered/conf.xml

@@ -929,7 +929,7 @@
             containing valid nodes.
         
     -->
-    <xupdate allowed-fragmentation="50000" enable-consistency-checks="no"/>
+    <xupdate allowed-fragmentation="-1" enable-consistency-checks="no"/>
 
     <!-- 
         Load this configuration into Saxon