Make DB interactions in TESTS scoped

[AKlaus]

EF Core maintains an internal cache of records that may skew some test results when

• the DataContext runs a check against the local cache rather than the actual DB;
• the DataContext's cache is out-of-sync with the DB for some reason.

Steps to mitigate potential issues:

1. Use AsNoTracking() on querying data that don't require tracking.
2. Change the test assertion on running against a new instance of DataContext