Ensure enough nodes in WriteLoadForecasterIT

These tests would sometimes create 1-replica indices in a 1-node cluster
which will never reach `green` health. This commit ensures there are
enough nodes to allocate all the shards.

Closes elastic#133455
Closes elastic#134124
Closes elastic#134123
Backport of elastic#134132 to `9.1`

Author: DaveCTurner

muted-tests.yml

@@ -449,9 +449,6 @@ tests:
 - class: org.elasticsearch.xpack.kql.parser.KqlParserBooleanQueryTests
   method: testParseOrQuery
   issue: https://github.com/elastic/elasticsearch/issues/133863
-- class: org.elasticsearch.xpack.writeloadforecaster.WriteLoadForecasterIT
-  method: testWriteLoadForecastGetsPopulatedDuringRollovers
-  issue: https://github.com/elastic/elasticsearch/issues/134123
 
 # Examples:
 #

x-pack/plugin/write-load-forecaster/src/internalClusterTest/java/org/elasticsearch/xpack/writeloadforecaster/WriteLoadForecasterIT.java

@@ -145,6 +145,8 @@ private void setUpDataStreamWriteDocsAndRollover(String dataStreamName) throws E
     private void setUpDataStreamWriteDocsAndRollover(String dataStreamName, Settings extraIndexTemplateSettings) throws Exception {
         final int numberOfShards = randomIntBetween(1, 5);
         final int numberOfReplicas = randomIntBetween(0, 1);
+        internalCluster().ensureAtLeastNumDataNodes(numberOfReplicas + 1);
+
         final Settings indexSettings = Settings.builder()
             .put(extraIndexTemplateSettings)
             .put(IndexMetadata.SETTING_NUMBER_OF_SHARDS, numberOfShards)