NetworkController tests are very slow and should run faster

Specifically, these test files are slow to run:

- `NetworkController.test.ts` (~32s)
- `create-network-client.test.ts` (~311s !!)

We should investigate why it takes so long for these to run, and fix them.

I have a hunch that it has something to do with the custom assertions that I added which wait for promises to be resolved, but there may be other reasons.

## Acceptance Criteria

- All tests for `network-controller` should run in less than 10 seconds.

Start by running NetworkController.test.ts and create-network-client.test.ts and measuring their current runtimes. Inspect the custom assertions that wait for promises, along with other causes of delay, then rerun all network-controller tests; done means they complete in under 10 seconds.

Written by the indexing model from the issue text.