Fix flaky test TestCollapseQParserPlugin.testMultiSort (#3609)

Relying on index order is unreliable.

Author: dsmiley

solr/core/src/test/org/apache/solr/search/TestCollapseQParserPlugin.java

@@ -107,17 +107,6 @@ public void testMultiSort() {
         "//result/doc[1]/str[@name='id'][.='8']",
         "//result/doc[2]/str[@name='id'][.='4']");
 
-    // tie broken by index order
-    params = new ModifiableSolrParams();
-    params.add("q", "*:*");
-    params.add("fq", "{!collapse field=group_s sort='test_l desc'}");
-    params.add("sort", "id_i desc");
-    assertQ(
-        req(params),
-        "*[count(//doc)=2]",
-        "//result/doc[1]/str[@name='id'][.='6']",
-        "//result/doc[2]/str[@name='id'][.='2']");
-
     // score, then tiebreakers; note top level sort by score ASCENDING (just for weirdness)
     params = new ModifiableSolrParams();
     params.add("q", "*:* term_s:YYYY");