refactor(assets-controllers): Simplify TokenBalancesController tests

[Gudahtt]

Explanation

The TokenBalancesController tests have been refactored to use mocks instead of NetworkController and PreferencesController instances. This should make the tests easier to read, and it decouples the tests further from the other controllers.

References

Relates to #3708

Changelog

@metamask/assets-controllers

Added

• Add getDefaultTokensState function to the TokensController

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've highlighted breaking changes using the "BREAKING" category above as appropriate

[jiexi]

If we're going to use this pattern in more of the tests, is it worth creating some sort of helper that returns fns that keeps track of the listener and a way to trigger them?

const mockStateChangeListener<T>() => {
  const listeners: ((state: T) => void)[] = [];
  
  return {
    onStateChange: (listener) => {
          listeners.push(listener);
    },
     triggerStateChange: () => {
         for (const listener of listeners) {
          listener(state);
        }
     }
  }
}

Maybe? Not sure if it's complicated enough to warrant the helper though

[Gudahtt]

Good suggestion @jiexi . That sounds like it could be useful, but we may see this pattern used less often as we complete the BaseControllerV2 migrations so maybe the need for it will be eliminated fairly soon.

The BaseControllerV2 controllers will use the messenger rather than on[event] constructor parameters.