this is the smallest real example probably: https://github.com/pantsbuild/pants/blob/7346b0b964267daac15626ae4d7b7cf6dacd9267/tests/python/pants_test/engine/test_build_files.py#L29-L36

that represents the absolute most unity a unit test of a rule can be

ConsoleRuleTestBase is closer to an integration test

self.context.scheduler.product_request

on a TestBase subclass is a middle ground where you can install additional rules and shims, but still operate on a bunch of rules