NetworkController tests are very slow and should run faster

[mcmire]

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.