feat: Add Test Case Generation for Weak Isolation Levels in Different Databases

[WeihuaSun]

issue #42
Generate different test cases for weak isolation levels of different databases.

The isolation level of the database has different capabilities to break the cycles in POP. If the isolation level can break the cycles in POP, then the test case corresponding to POP should not have data anomalies under the isolation level. I corresponded the isolation level with the ability to break the cycles and summarized 6 mechanisms.

By specifying the database and isolation level, the code can generate test cases for the isolation level.

[tencent-adm]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}