I don't know what they do at Shopify, but I have seen teams be successful with using dbt's (https://getdbt.com) unit testing methodologies and tooling. It makes it very easy to do things like cardinality and null checking, and if you're using a datamart like Snowflake with snapshots, it is super easy to have test harnesses with representative data. To me the combination of tools there is the secret sauce, since data mocking is such a giant tedious pain in the ass.