cover all possible scenarios in upgrading tests

[lance6716]

Feature Request

Is your feature request related to a problem? Please describe:

In order to avoid my bug like https://github.com/pingcap/dm/issues/1644 happening again in future, we should cover every possible scenarios before upgrading, and check main commands works well after upgrading.

Describe the feature you'd like:

we should cover

• [x] pessimistic lock not resolved
• [x] optimistic lock not resolved
• [ ] online DDL not finished
• [x] start-task could continue from checkpoint at sync stage
• [ ] start-task from checkpoint at load stage (not sure)
• [x] pause-task will not resume automaticlly, and resume-task could work
• [ ] relay enabled
• [x] MariaDB

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

[lance6716]

please help add more scenarios @GMHDBJD @lichunzhu @glorv @Ehco1996