You cannot sc:SplitFieldValue on a rendering parameter (e.g. a multilist of items) because that method takes a field name rather than a comma-delimited string of IDs. To split a rendering parameter string, use sc:SplitValue:

<xsl:for-each select="sc:SplitValue($testitems, '|')">
      <xsl:variable name="myitem" select="sc:item(text(),.)" />
      <xsl:value-of select="$myitem/@name"/>
</xsl:for-each>