HBASE-28133 TestSyncTimeRangeTracker fails with OOM with small -Xms values (#5450)

reduce test dataset size by 20% to fit heap with safety margin

Signed-off-by: Duo Zhang <zhangduo@apache.org>
Signed-off-by: Peter Somogyi <psomogyi@apache.org>
(cherry picked from commit 0d15d03)

Author: stoty

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

@@ -34,7 +34,7 @@ public class TestSyncTimeRangeTracker extends TestSimpleTimeRangeTracker {
   public static final HBaseClassTestRule CLASS_RULE =
     HBaseClassTestRule.forClass(TestSyncTimeRangeTracker.class);
 
-  private static final int NUM_KEYS = 10000000;
+  private static final int NUM_KEYS = 8000000;
   private static final int NUM_OF_THREADS = 20;
 
   @Override