Since this test groups by a constant numeric value, the use of a collation is unnecessary, and so an implementation can avoid triggering the error case which this test aims to hit.

   <xsl:for-each-group select="$pop/*"
                             group-adjacent="1"
                             collation="http://unknown.collation.uri/">
            <xsl:value-of select="current-group()"/>
         </xsl:for-each-group>


I'd suggest changing the test to group by a non-constant string.  e.g.

   <xsl:for-each-group select="$pop/*"
                             group-adjacent="local-name(.)"
                             collation="http://unknown.collation.uri/">
            <xsl:value-of select="current-group()"/>
         </xsl:for-each-group>