feat: use :memory: as the unit state storage location for ops.testing (…

…#1494)

This PR changes Scenario to always use `:memory:` as the location for
the unit state database, to speed up running tests.

In most cases, the charm root location is a `TemporaryDirectory`, so the
unit state database only exists during the event emission anyway. The
database could be accessed if a specific root directory was provided (or
inside the context manager context) but the correct way to interact with
the database is through the State API (deferred events, saved state
objects) so we don't want to leave that open for users.

Timing (best of 3):

|Suite|main|branch|
|-|-|-|
|operator unit tests (no concurrency)|4m11.923s|3m41.354s|
|traefik scenario tests|3m49.479s|1m56.795s|
|kafka-k8s-operator scenario tests|0m13.463s|0m13.236s|

Note that this [is already the case for
Harness](https://github.com/canonical/operator/blob/0ad731a6a9118dfac240e84681f5c41810c4f3f2/ops/_private/harness.py#L309).

Refs #1434

testing/src/scenario/_ops_main_mock.py

@@ -143,9 +143,7 @@ def _setup_root_logging(self):
 
     def _make_storage(self, _: _Dispatcher):
         # TODO: add use_juju_for_storage support
-        # TODO: Pass a charm_state_path that is ':memory:' when appropriate.
-        charm_state_path = self._charm_root / self._charm_state_path
-        storage = ops.storage.SQLiteStorage(charm_state_path)
+        storage = ops.storage.SQLiteStorage(":memory:")
         logger.info("Copying input state to storage.")
         self.store = UnitStateDB(storage)
         self.store.apply_state(self.state)