Mysql CDC test fix: SET TRANSACTION_ISOLATION TO SERIALIZABLE

[patrickwwbutler]

Fixes https://github.com/MaterializeInc/database-issues/issues/11264

This test was flaky because sometime the select * from t1 query would run after the mysql db was finished restoring, and so no more transactions occur, and thus the progress never ticks forward, and our query is forever waiting for the next transaction so it can be sure the current one is complete.

Running SET TRANSACTION_ISOLATION TO SERIALIZABLE fixes this by not waiting for the next transaction.

[github-actions]

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[martykulma]

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