HDFS-16184. De-flake TestBlockScanner#testSkipRecentAccessFile (#3329)

Reviewed-by: Ayush Saxena <ayushsaxena@apache.org>
Signed-off-by: Takanobu Asanuma <tasanuma@apache.org>

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/datanode/TestBlockScanner.java

@@ -1004,6 +1004,8 @@ public class TestBlockScanner {
         TestScanResultHandler.getInfo(ctx.volumes.get(0));
     synchronized (info) {
       info.shouldRun = true;
+      info.sem = new Semaphore(1);
+      info.sem.acquire();
       info.notify();
     }
     try {
@@ -1017,6 +1019,7 @@ public class TestBlockScanner {
       LOG.debug("Timeout for all files are accessed in last period.");
     }
     synchronized (info) {
+      info.sem.release();
       info.shouldRun = false;
       info.notify();
     }