add E2E test cases for "scale in tikv simultaneously"

[csuzhangxc]

Feature Request

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

"scale in tikv simultaneously" supported in #3881.

Describe the feature you'd like:

We need to add some E2E test cases for this feature. at least the bold cases in the test plan

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

[csuzhangxc]

@duduainankai Would you like to help with this?

[lizhemingi]

@duduainankai Would you like to help with this?

Yeah, of course.

[lizhemingi]

/assign