Add basic test for IsActive

[AlpinYukseloglu]

Closes: #1904

What is the purpose of the change

This PR adds a test that (trivially) checks to make sure that IsActive returns true for balancer pools. The primary purpose of this change is to make sure that if IsActive is ever used as an emergency switch, it is not accidentally left off for any or all pools.

I realize that my implementation has more structure than might seem necessary for a test this simple, but given how we intend to add more complex mock tests to cover IsActive flipping functionality in the future (#1904 (comment)), I figured setting it up properly the first time around might make it easier to implement these other tests later.

Brief Changelog

• Add a test that makes sure IsActive always returns true

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? (no)
• Is a relevant changelog entry added to the Unreleased section in CHANGELOG.md? (no)
• How is the feature or change documented? (not documented)