[stableswap]: Add checks for invalid cfmm inputs

[AlpinYukseloglu]

Closes: #2694

What is the purpose of the change

From issue:

While testing our stableswap CFMMs (both two-asset and multi), I noticed that given their complexity it is possible for certain really weird inputs to get passed into the solvers that go completely uncaught. Both for security reasons and for more time-effective testing/bug squashing, I think we should implement validity checks for inputs to (and outputs of) the CFMM solvers.

This PR adds these validity checks.

Brief Changelog

• Ensure inputs and outputs of CFMM functions are checked for basic validity (not negative, outputs not greater than reserves etc.)

Testing and Verifying

This change added tests that verify negative input checks in stableswap/amm_test.go

Documentation and Release Note

• Does this pull request introduce a new feature or user-facing behavior changes? (no)
• Is a relevant changelog entry added to the Unreleased section in CHANGELOG.md? (no)
• How is the feature or change documented? (not documented)

[ValarDragon]

nice, ty!

[AlpinYukseloglu]

I just went ahead and refactored the tests to fit our standards – I figured it would be easier to resolve issues in all the open PRs right now than try to fit in a full test refactor later