Port context unit tests

[hownowstephen]

Provide a context-like object and port unit tests https://github.com/Shopify/liquid/blob/3146d5c3f26ca5f6f8cf58efd5562a5bb0b3c57b/test/unit/context_unit_test.rb

[ghost]

Working on this now since the Context is pretty central. Full impl requires figuring out how the Strainer is going to work since it relies on it, but seems like that part should be pretty easy to add after the rest of the mechanics are worked out.