do not generate the unsupported test scenario

[cfzjywxk]

In the issue pingcap/tidb#20909.
If the conflict value generting in a single DML transaction and the conflict value column has concurrent add unique index column operations, the drainer will fail to replicate.

Need to avoid generate the unsupport test schenario that conflict value generating in a DML transaction whose index column has concurrent add unique index.

[cfzjywxk]

@vivid392845427
Could check about this issue if there is time, feel free to ask any questions.