Fix unit test with _all:* query

Author: jimczi

core/src/test/java/org/elasticsearch/index/query/QueryStringQueryBuilderTests.java

@@ -869,7 +869,7 @@ public void testExistsFieldQuery() throws Exception {
             fieldQueries.add(new ConstantScoreQuery(new TermQuery(new Term("_field_names", type))));
         }
         expected = new DisjunctionMaxQuery(fieldQueries, 0f);
-        Query expectedSingle = new ConstantScoreQuery(new TermQuery(new Term("_field_names", "_all")));
+        Query expectedSingle = new MatchAllDocsQuery();
         assertThat(query, anyOf(equalTo(expected), equalTo(expectedSingle)));
     }