In order to be able to better compare differences between the version of
NetworkController in this repo and the version in the `core` repo before
we replace this version with the `core` version, this commit converts
the NetworkController network client tests to TypeScript.
The added types here are copied from the `core` repo. We plan on
making more improvements on the `core` side at some point to polish the
tests and types and reduce some of the duplication, but for now we're
just trying to keep things as similar as possible.
