TEST: avoid merge in testSegmentMemoryTrackedInBreaker

This commit indexes an extra document to avoid triggering merges.

Relates LUCENE-8263

server/src/test/java/org/elasticsearch/index/shard/IndexShardTests.java

@@ -2979,6 +2979,7 @@ public class IndexShardTests extends IndexShardTestCase {
         assertThat(breaker.getUsed(), greaterThan(preRefreshBytes));
 
         indexDoc(primary, "_doc", "4", "{\"foo\": \"potato\"}");
+        indexDoc(primary, "_doc", "5", "{\"foo\": \"potato\"}");
         // Forces a refresh with the INTERNAL scope
         ((InternalEngine) primary.getEngine()).writeIndexingBuffer();