[x/gamm][stableswap]: Add tests for single-asset joins (`calcSingleAssetJoinShares`)

[AlpinYukseloglu]

Background

Our current implementation of calcSingleAssetJoinShares is untested. We should make tests that cover similar cases to what we have in balancer.

Suggested Design

• Create single-asset pool join tests that mirror what we have in balancer but with our stableswap CFMM

Acceptance Criteria

• All tests pass and removing updatePoolForJoin through mutation testing doesn't break calcSingleAssetJoinShares

[pysel]

Is this issue available to work on?

[p0mvn]

@RusAkh Yes, please

You can get inspiration from the existing test cases in balancer. However, they might not be exhaustive so if you think of any extra, please add them

[AlpinYukseloglu]

I actually covered a significant portion of this issue in #2905, so it might not be a great one to pick up if you don't have context (properly testing this functionality is actually a relatively complicated process)

[AlpinYukseloglu]

Update: single-asset joins are now tested (#2905) and can be further tested as part of #2900 and #2346