Optimize test case in migrating slots

[haorenfsa]

Status

DEVELOPMENT

Description

Add test case when the key being queried is migrating.

Todos

• [x] Unit Tests

It will be awesome if you cover the negative test cases as well.

Notes to reviewers

Review the following things, including but not limit to:

1. Test cases (This is a MUST, don't pass the PR if it's missing)
2. Design
3. Code (styles, magic...)