Save duplicate REST client in ESRestTestCase (#117910)

I debugged some tests today and noticed that these two clients
are the same in almost all cases, no need to use extra connections.
Might give usa small speedup for these tests that tend to be quite slow
relative to the node client based tests.

test/framework/src/main/java/org/elasticsearch/test/rest/ESRestTestCase.java

@@ -333,8 +333,11 @@ public void initClient() throws IOException {
             assert testFeatureServiceInitialized() == false;
             clusterHosts = parseClusterHosts(getTestRestCluster());
             logger.info("initializing REST clients against {}", clusterHosts);
-            client = buildClient(restClientSettings(), clusterHosts.toArray(new HttpHost[clusterHosts.size()]));
-            adminClient = buildClient(restAdminSettings(), clusterHosts.toArray(new HttpHost[clusterHosts.size()]));
+            var clientSettings = restClientSettings();
+            var adminSettings = restAdminSettings();
+            var hosts = clusterHosts.toArray(new HttpHost[0]);
+            client = buildClient(clientSettings, hosts);
+            adminClient = clientSettings.equals(adminSettings) ? client : buildClient(adminSettings, hosts);
 
             availableFeatures = EnumSet.of(ProductFeature.LEGACY_TEMPLATES);
             Set<String> versions = new HashSet<>();