tests: add govshuttle tests

[sairamplex]

Description

• add tests to functions in proposal.go

• add tests to functions in keeper.go

Author Checklist

All items are required. Please add a note to the item if the item is not applicable and please add links to any relevant follow up issues.

I have...

• [x] targeted the correct branch (see PR Targeting)
• [ ] provided a link to the relevant issue or specification
• [x] included the necessary unit and integration tests
• [x] reviewed "Files changed" and left comments if necessary

Reviewers Checklist

All items are required. Please add a note if the item is not applicable and please add your handle next to the items reviewed if you only reviewed selected items.

I have...

• [ ] confirmed all author checklist items have been addressed
• [ ] reviewed state machine logic
• [ ] reviewed API design and naming
• [ ] reviewed documentation is accurate
• [ ] reviewed tests and test coverage
• [ ] manually tested (if applicable)