HBASE-19658 make the test testFlatteningToJumboCellChunkMap() stable, by eliminating the possibility of third cell to be added while in-memory-flush is still in progress

hbase-server/src/test/java/org/apache/hadoop/hbase/regionserver/TestCompactingToCellFlatMapMemStore.java

@@ -807,12 +807,17 @@ public class TestCompactingToCellFlatMapMemStore extends TestCompactingMemStore
 
     memstore.clearSnapshot(snapshot.getId());
 
-    String[] keys2 = { "C", "D", "E"};
+    // Allocating two big cells (too big for being copied into a regular chunk).
+    String[] keys2 = {"C", "D"};
     addRowsByKeys(memstore, keys2, val);
     while (((CompactingMemStore) memstore).isMemStoreFlushingInMemory()) {
       Threads.sleep(10);
     }
-    totalHeapSize = 1 * oneCellOnCSLMHeapSize + MutableSegment.DEEP_OVERHEAD
+
+    // The in-memory flush size is bigger than the size of a single cell,
+    // but smaller than the size of two cells.
+    // Therefore, the two created cells are flattened together.
+    totalHeapSize = MutableSegment.DEEP_OVERHEAD
             + CellChunkImmutableSegment.DEEP_OVERHEAD_CCM
             + 2 * oneCellOnCCMHeapSize;
     assertEquals(totalHeapSize, ((CompactingMemStore) memstore).heapSize());