x/gamm: Add overflow tests to balancer spot price

[AlpinYukseloglu]

Closes: #2207

What is the purpose of the change

Tests for understanding spot price bounds for balancer pools.

Brief Changelog

• New tests for SpotPrice()

Testing and Verifying

This change is a trivial rework / code cleanup without any test coverage.

Documentation and Release Note

• Does this pull request introduce a new feature or user-facing behavior changes? (yes / no)
• Is a relevant changelog entry added to the Unreleased section in CHANGELOG.md? (yes / no)
• How is the feature or change documented? (not applicable / specification (x/<module>/spec/) / Osmosis docs repo / not documented)