[Rollup] Fix duplicate field names in test (#32075)

This commit ensures that random field names do not clash with the explicit field names
set by the tests.

Closes #32067

x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/rollup/ConfigTestHelpers.java

@@ -94,7 +94,7 @@ public static DateHistoGroupConfig.Builder getDateHisto() {
         if (ESTestCase.randomBoolean()) {
             dateHistoBuilder.setDelay(new DateHistogramInterval(randomPositiveTimeValue()));
         }
-        dateHistoBuilder.setField(ESTestCase.randomAlphaOfLengthBetween(1, 10 ));
+        dateHistoBuilder.setField(ESTestCase.randomAlphaOfLengthBetween(5, 10));
         return dateHistoBuilder;
     }
 
@@ -112,8 +112,8 @@ public static TermsGroupConfig.Builder getTerms() {
     }
 
     public static  List<String> getFields() {
-        return IntStream.range(0, ESTestCase.randomIntBetween(1,10))
-                .mapToObj(n -> ESTestCase.randomAlphaOfLengthBetween(1,10))
+        return IntStream.range(0, ESTestCase.randomIntBetween(1, 10))
+                .mapToObj(n -> ESTestCase.randomAlphaOfLengthBetween(5, 10))
                 .collect(Collectors.toList());
     }