init a list with only one element with singleton function to enhance perf (#9208)

Reviewers: Bill Bejeck <bbejeck@apache.org>

streams/src/test/java/org/apache/kafka/streams/processor/internals/ProcessorNodeTest.java

@@ -16,7 +16,6 @@
  */
 package org.apache.kafka.streams.processor.internals;
 
-import java.util.Arrays;
 import java.util.Properties;
 import org.apache.kafka.common.metrics.Metrics;
 import org.apache.kafka.common.serialization.StringSerializer;
@@ -169,7 +168,7 @@ public class ProcessorNodeTest {
 
         builder.<String, String>stream("streams-plaintext-input")
             .flatMapValues(value -> {
-                return Arrays.asList("");
+                return Collections.singletonList("");
             });
         final Topology topology = builder.build();
 

streams/src/test/java/org/apache/kafka/streams/state/internals/FilteredCacheIteratorTest.java

@@ -25,6 +25,7 @@ import org.apache.kafka.test.GenericInMemoryKeyValueStore;
 import org.junit.Before;
 import org.junit.Test;
 
+import java.util.Collections;
 import java.util.List;
 
 import static java.util.Arrays.asList;
@@ -121,7 +122,7 @@ public class FilteredCacheIteratorTest {
     @Test
     public void shouldFilterEntriesNotMatchingHasNextCondition() {
         final List<KeyValue<Bytes, LRUCacheEntry>> keyValues = toList(firstEntryIterator);
-        assertThat(keyValues, equalTo(asList(firstEntry)));
+        assertThat(keyValues, equalTo(Collections.singletonList(firstEntry)));
     }
 
     @Test(expected = UnsupportedOperationException.class)