Summary: `assert` function added. Useful to us for asserting DB state in harness tests. Potentially useful to the client for breaking out of a query as soon as a predicate fails, as opposed to collecting result and checking them client-side. Reviewers: buda, mislav.bradac, teon.banek Reviewed By: teon.banek Subscribers: pullbot Differential Revision: https://phabricator.memgraph.io/D750