refactor(staking): add missing msgServer tests and move ValidateBasic() logic to msgServer

[likhita-809]

Description

ref: #15648
This PR moves ValidateBasic() logic to msgServer and adds missing msgServer tests

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[julienrbrt]

Crazy that msg server wasn't tested. good catch!
I think we should not completely delete ValidateBasic logic when it is used at multiple place. I would rename it for Validate instead (so it does not match the interface but we can still avoid code duplication and possible future discrepancies). This case is only valid for the MsgCreateValidator that we need to check at 3 places.

[julienrbrt]

let's move the cheap checks before any keeper calls.

comments

[julienrbrt]

lgtm!