Tests: Add in-memory database for testing

Elwig/Helpers/AppDbUpdater.cs

@@ -8,6 +8,7 @@ using System.Threading.Tasks;
 namespace Elwig.Helpers {
     public static class AppDbUpdater {
 
+        // Don't forget to update value in Tests/fetch-resources.bat!
         public static readonly int RequiredSchemaVersion = 12;
 
         private static int VersionOffset = 0;