loaders.tests: Simplify and add coverage to check-snapshot
Tests are running with pg-storage instance. So we never pass there. Also this method will get improved upon soon [1]
- [1] T2483
This adds the necessary scenario to check the "no snapshot exist with that id" case.
Related to T2486
Test Plan
tox
Migrated from D3444 (view on Phabricator)