Update TokenListController tests to not create networkController inst… (

#1061)

**No NetworkController in tests**

**Description**:
Mocking the behaviour of the network controller makes the tests more
flexible, and lessens the burden of cleanup / maintainability of the
tests. This PR removes the `NetworkController` from the
`TokenListController` tests.

Relates to #1051

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Co-authored-by: Mark Stacey <markjstacey@gmail.com>