Separate the 'context' and 'side effects' aspects of the testing context

[tonyandrewmeyer]

The context has a bunch of "this is the context the event runs in":

• charm_spec
• charm_root (in terms of what's there when the event starts - anything the charm writes to it is more state)
• juju_version
• app_trusted

But there's also a lot of side-effects, which have grown noticeably in 7.x:

• juju_log
• app_status_history
• unit_status_history
• workload_version_history
• requested_storages
• exec_history (TBC)
• removed_secret_revisions
• emitted_events
• action_logs
• action_results

I feel like it would be cleaner if Context was more cleanly just the context (plus config, like the settings for capturing events), and the side effects were available some other way.

Moved from canonical/ops-scenario#171