[stableswap]: Add scaling factor tests

[AlpinYukseloglu]

Background

Scaling factors currently have a basic test skeleton commented out in the keeper tests that needs to be moved to our pool_test.go and filled with test cases for scaling/descaling pool assets

Suggested Design

• Move commented tests from keeper tests to pool_test.go
• Add thorough test cases (even/uneven pools, even/uneven scaling factors, min and max scaling factors, overflow/underflow, panic catching etc.)

Acceptance Criteria

• All new and existing tests pass

[AlpinYukseloglu]

@hieuvubk I saw that you self-assigned - I was working on this alongside a few other scaling factor changes so will unassign you for now (I hope you weren't too far into it!).

In general I will probably be picking up (or already have significant progress for) the stableswap-related issues I create unless I tag them as 'Good First Issue' :)

[hieuvubk]

Okay got it, thank you!