Fix flaky StampedWriterPreferredLock test

[Caideyipi]

What

• Make testAcquireReadLockWhileWriting wait until the reader is actually blocked by the held write lock before releasing it.
• Wait for the reader thread to finish with a latch and release the write lock in failure paths.

Testing

• mvn -pl iotdb-core/datanode spotless:apply
• javac --release 8 ... StampedWriterPreferredLock.java StampedWriterPreferredLockTest.java && java ... org.junit.runner.JUnitCore org.apache.iotdb.db.metadata.mtree.lock.StampedWriterPreferredLockTest

The full Maven datanode test command is currently blocked locally by unrelated datanode generated-source/test compilation errors.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 40.91%. Comparing base (a420681) to head (5363380).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@## master #17832 +/- ##
============================================
+ Coverage 40.87% 40.91% +0.04% 
Complexity 2611 2611 ============================================
Files 5186 5187 +1 Lines 351308 351561 +253 Branches 44971 45008 +37 ============================================
+ Hits 143585 143854 +269 + Misses 207723 207707 -16 

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