Add unit test for group score calc to assign replicas evenly

[ipsum-0320]

What type of PR is this? /kind feature

What this PR does / why we need it: Optimize the score calculation method to better measure the pros and cons of the group. This PR added unit test for group score calc to assign replicas evenly.

Which issue(s) this PR fixes: Fixes #4805

Special notes for your reviewer: @whitewindmills

Does this PR introduce a user-facing change?:

`karmada-scheduler`: implement group score calculation instead of take the highest score of clusters