Fix search template request (#43509)

A seed was hit in (#43157) that caused mutateInstance to generate an identical 
instance. This change prevents that.

Author: jdconrad

modules/lang-mustache/src/test/java/org/elasticsearch/script/mustache/SearchTemplateRequestTests.java

@@ -63,8 +63,8 @@ protected SearchTemplateRequest mutateInstance(SearchTemplateRequest instance) t
         mutators.add(request -> request.setExplain(!request.isExplain()));
         mutators.add(request -> request.setSimulate(!request.isSimulate()));
 
-        mutators.add(request -> request.setRequest(
-            RandomSearchRequestGenerator.randomSearchRequest(SearchSourceBuilder::searchSource)));
+        mutators.add(request -> request.setRequest(randomValueOtherThan(request.getRequest(),
+                () -> RandomSearchRequestGenerator.randomSearchRequest(SearchSourceBuilder::searchSource))));
 
         SearchTemplateRequest mutatedInstance = copyInstance(instance);
         Consumer<SearchTemplateRequest> mutator = randomFrom(mutators);