[x/gamm][Testing] balancer/CalcJoinPoolShares

[p0mvn]

Background

We have observed rounding errors in balancer post-refactor (#1307). There are several balancer pool methods being called from the function in #1307 under the hood that are not unit-tested.

As a result, it is non-trivial to identify the source of the problem. All methods should be covered by unit tests. One of such untested methods is CalcJoinPoolShares

On initial investigation, this method does many things, violating the single responsibility principle in SOLID. As a result, it is difficult to reason about and test.

The person taking on this issue should investigate if it is worth refactoring this method to extract a few others to be called under the hood to mitigate the violation. If any, all newly extracted methods should be unit tested, including CalcJoinPoolShares itself.

Acceptance Criteria

• investigate the need for extracting several methods from CalcJoinPoolShares to ease unit testing and improve readability
• ensure all logic in CalcJoinPoolShares and newly extracted functions/methods, if any, are covered by unit tests
• edge cases are well thought out
• all old and new unit tests pass

[p0mvn]

@alexanderbez would you be interested in working on this? Thanks for offering to help with the tests