Mocha: Set up helper functions for creating variants of test fixtures

We have a few `create()` helpers [already](https://github.com/search?q=repo%3Aconfluentinc%2Fvscode+path%3A%2F%5Etests%5C%2Funit%5C%2FtestResources%5C%2F%2F+%22export+function+create%22&type=code), but most tests are taking the existing `TEST_` and explicitly creating new instances and/or mocking out entirely new objects. I think we could do a lot of cleanup here for consistency so new tests (and/or Copilot) referencing existing tests can follow a single pattern.

Could be done at the same time as https://github.com/confluentinc/vscode/issues/2044