Mock network requests within all TransactionController tests

[mcmire]

This issue is part of mocking network requests across all tests.

[spiritanand]

Interested. Please assign.

[mcmire]

Hi @spiritanand. We tend to assign tickets as a part of our development process when we've brought them into our roadmap, but we'd gladly take a PR if this is something you'd like to work on. You can see an example of a previous PR addressing this ticket here: #916

[spiritanand]

I will wait for it to be added @mcmire. If there are any issues which you feel I can look at, please let me know.

Thanks