Allow contexts to be used in unit testing with FakeUnleash

[markusjevringflowers]

Describe the feature request

I would like to be able to pass an UnleashContext to FakeUnleash.enable(), so that it matches Unleash.isEnabled()

Background

You have the FakeUnleash for unit testing, which is great. However, for things like dependency injection, and per your documentation, you will often end up with a singleton instance of FakeUnleash. Especially when you are re-using the spring context between tests. If the tests are single-threaded, this fine, as you can mutate global state without fear of interaction. However, with concurrent tests, mutating global state like this can lead to incorrect interactions.

Solution suggestions

Make FakeUnleash track and use UnleashContext so that FakeUnleash.enable() matches Unleash.isEnabled()

[gastonfournier]

Thanks, it seems reasonable, if you'd like, feel free to open a PR with your suggestion.