HDFS-17846. Enhance the stability of the unit test TestDirectoryScanner.

[huangzhaobo99]

Description of PR

JIRA: https://issues.apache.org/jira/browse/HDFS-17846

We need lots of blocks so the report compiler threads have enough to keep them busy while we watch them.
The number of blocks: 20000->40000.

[hadoop-yetus]

💔 -1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	0m 53s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 0s		No case conflicting files found.
+0 🆗	codespell	0m 0s		codespell was not available.
+0 🆗	detsecrets	0m 0s		detect-secrets was not available.
+1 💚	@author	0m 0s		The patch does not contain any @author tags.
+1 💚	test4tests	0m 0s		The patch appears to include 1 new or modified test files.
			_ trunk Compile Tests _
-1 ❌	mvninstall	4m 20s	/branch-mvninstall-root.txt	root in trunk failed.
+1 💚	compile	1m 38s		trunk passed with JDK Ubuntu-11.0.27+6-post-Ubuntu-0ubuntu120.04
+1 💚	compile	1m 8s		trunk passed with JDK Private Build-1.8.0_452-8u452-gaus1-0ubuntu120.04-b09
+1 💚	checkstyle	0m 52s		trunk passed
+1 💚	mvnsite	1m 21s		trunk passed
+1 💚	javadoc	1m 5s		trunk passed with JDK Ubuntu-11.0.27+6-post-Ubuntu-0ubuntu120.04
+1 💚	javadoc	1m 33s		trunk passed with JDK Private Build-1.8.0_452-8u452-gaus1-0ubuntu120.04-b09
+1 💚	spotbugs	3m 7s		trunk passed
+1 💚	shadedclient	43m 5s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	1m 11s		the patch passed
+1 💚	compile	1m 23s		the patch passed with JDK Ubuntu-11.0.27+6-post-Ubuntu-0ubuntu120.04
+1 💚	javac	1m 23s		the patch passed
+1 💚	compile	1m 7s		the patch passed with JDK Private Build-1.8.0_452-8u452-gaus1-0ubuntu120.04-b09
+1 💚	javac	1m 7s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	checkstyle	0m 47s		the patch passed
+1 💚	mvnsite	1m 15s		the patch passed
+1 💚	javadoc	0m 58s		the patch passed with JDK Ubuntu-11.0.27+6-post-Ubuntu-0ubuntu120.04
+1 💚	javadoc	1m 31s		the patch passed with JDK Private Build-1.8.0_452-8u452-gaus1-0ubuntu120.04-b09
+1 💚	spotbugs	3m 16s		the patch passed
+1 💚	shadedclient	42m 59s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 💚	unit	516m 27s		hadoop-hdfs in the patch passed.
+1 💚	asflicense	0m 55s		The patch does not generate ASF License warnings.
		629m 17s

Subsystem	Report/Notes
Docker	ClientAPI=1.51 ServerAPI=1.51 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8022/1/artifact/out/Dockerfile
GITHUB PR	#8022
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient spotbugs checkstyle codespell detsecrets
uname	Linux 1c0d59c6a1ff 5.15.0-144-generic #157-Ubuntu SMP Mon Jun 16 07:33:10 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / ba617a3
Default Java	Private Build-1.8.0_452-8u452-gaus1-0ubuntu120.04-b09
Multi-JDK versions	/usr/lib/jvm/java-11-openjdk-amd64:Ubuntu-11.0.27+6-post-Ubuntu-0ubuntu120.04 /usr/lib/jvm/java-8-openjdk-amd64:Private Build-1.8.0_452-8u452-gaus1-0ubuntu120.04-b09
Test Results	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8022/1/testReport/
Max. process+thread count	2234 (vs. ulimit of 5500)
modules	C: hadoop-hdfs-project/hadoop-hdfs U: hadoop-hdfs-project/hadoop-hdfs
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8022/1/console
versions	git=2.25.1 maven=3.9.11 spotbugs=4.2.2
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