Uniter context: export FlushContext

Most of the tests for finalizeContext, which is *complex* (not to mention complicated...) were invoking it by side-effect. So, it was really not well tested at all; to rectify this:

* context.Runner now uses a context.Context interface, not an explicit *HookContext
  * the finalizeContext method became FlushContext
  * the hookVars func became a HookVars method
  * the context ActionData method replaces an ad-hoc combination of similarly horrible access methods
* the various flush tests that depended on running a fake hook are now calling FlushContext directly; and slightly more than before, although (tbh) not *enough* by a long shot
* context.Runner.Run* methods are now tested using a mock context.Context, so we now have *actual tests* that the contexts get flushed
  * makeCharm/hookSpec now creates scripts in arbitrary dirs, so we can actually *test running an action* /grrmbl