Ignore assertions element is test-case node.

nunit3-junit/nunit3-junit.xslt

@@ -59,6 +59,9 @@
     <skipped message="{./message}"/>
   </xsl:template>
   
+  <xsl:template match="test-case/assertions">
+  </xsl:template>
+
   <xsl:template match="test-suite/reason"/>
 
   <xsl:template match="properties"/>