Write test for periodic vesting msg

[amaury1093]

Summary

Add tests for periodic vesting msg #9596

Problem Definition

We added a new msg for periodic vesting accounts, but there we no tests ref #9596 (comment).

Proposal

• CLI tests
• Msg grpc server tests

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[JimLarson]

FYI, I'm writing some of these tests in the Agoric fork of cosmos-sdk. I'll share progress here and submit a PR for this repo when I'm done. (I'm out for the next 2 weeks, though.)

[alexanderbez]

Thank you @JimLarson!