[CI][Transform] fix GroupByOptimizerTests randomization failure part 2 (#76009)

avoid clashing field name by using a unique prefix

fixes #75957

Author: Hendrik Muhs

x-pack/plugin/transform/src/test/java/org/elasticsearch/xpack/transform/transforms/pivot/GroupByOptimizerTests.java

@@ -43,13 +43,13 @@ public void testOneGroupBy() {
     public void testOrderByType() {
         Map<String, SingleGroupSource> groups = new LinkedHashMap<>();
 
-        groups.put("terms1", randomTermsGroupSourceNoScript());
-        groups.put("date1", randomDateHistogramGroupSourceNoScript());
-        groups.put("terms2", randomTermsGroupSourceNoScript());
-        groups.put("date2", randomDateHistogramGroupSourceNoScript());
-        groups.put("hist1", randomHistogramGroupSourceNoScript());
+        groups.put("terms1", randomTermsGroupSourceNoScript("t1"));
+        groups.put("date1", randomDateHistogramGroupSourceNoScript("d1"));
+        groups.put("terms2", randomTermsGroupSourceNoScript("t2"));
+        groups.put("date2", randomDateHistogramGroupSourceNoScript("d2"));
+        groups.put("hist1", randomHistogramGroupSourceNoScript("h1"));
         groups.put("geo1", randomGeoTileGroupSource());
-        groups.put("hist2", randomHistogramGroupSourceNoScript());
+        groups.put("hist2", randomHistogramGroupSourceNoScript("h1"));
 
         List<String> groupNames = GroupByOptimizer.reorderGroups(Collections.unmodifiableMap(groups), Collections.emptySet())
             .stream()