testing: allow Hypothesis tests to disable extensions Doing this required the introduction of a mechanism for keeping track of more general config in the test. At present this is only used for extensions but it could be used more widely (e.g. to control specific extension behaviour) This greatly simplifies the extension management logic by introducing a general notion of config, which we maintain ourselves and pass to HG on every invocation. This results in significantly less error prone test generation, and also allows us to turn extensions off as well as on. The logic that used an environment variable to rerun the tests with an extension disabled now just edits the test file (in a fresh copy) to remove these --config command line flags.