[BUG] mvcc_trx_log_test occasional failed

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ref: https://github.com/oceanbase/miniob/actions/runs/10556062499/job/29240856790

[ RUN      ] MvccTrxLog.wal_rollback_abnormal
27: /home/runner/work/miniob/miniob/unittest/observer/mvcc_trx_log_test.cpp:711: Failure
27: Expected equality of these values:
27:   visible_count
27:     Which is: 499
27:   insert_num / 2
27:     Which is: 500
27: [  FAILED  ] MvccTrxLog.wal_rollback_abnormal (31718 ms)

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Start with unittest/observer/mvcc_trx_log_test.cpp around line 711 and run the MvccTrxLog.wal_rollback_abnormal test. Investigate why visible_count can be 499 instead of insert_num / 2 and reproduce the intermittent failure in the referenced CI environment. Done means the test no longer fails intermittently.