[tests] test additional_context passed to get_context
Now that we've added support for additional_context in gptty.context:get_context(), we should add some tests where we bootstrap additional context to the three different cases and validate the structure of the returned context, as well as its length.
Originally posted by @signebedi in https://github.com/signebedi/gptty/issues/37#issuecomment-1496220128