test(sns): Add unit tests for automatic upgrades triggered when targe…

…t version ahead of deployed version (#2784)

This PR contributes unit tests of the whole automatic-upgrade process
(as triggered by target_version being set). It adds a test for
Governance and Root.

## Why add unit tests when we already have integration tests?

Certain scenarios are much easier to test via unit testing. These tests
lay the groundwork for a test I want to write which will exercise edge
cases w.r.t. archive canisters, which are too hard to exercise in an
integration test

rs/sns/governance/src/governance.rs

@@ -6132,3 +6132,6 @@ mod fail_stuck_upgrade_in_progress_tests;
 
 #[cfg(test)]
 mod advance_target_sns_version_tests;
+
+#[cfg(test)]
+mod test_helpers;