Tests: Add in-memory database for testing

Elwig/Helpers/AppDbContext.cs

@@ -64,7 +64,8 @@ namespace Elwig.Helpers {
         public DateTime SavedLastWriteTime { get; private set; }
         public bool HasBackendChanged => SavedLastWriteTime != LastWriteTime;
 
-        public static string ConnectionString => $"Data Source=\"{App.Config.DatabaseFile}\"; Foreign Keys=True; Mode=ReadWrite; Cache=Default";
+        public static string? ConnectionStringOverride { get; set; } = null;
+        public static string ConnectionString => ConnectionStringOverride ?? $"Data Source=\"{App.Config.DatabaseFile}\"; Mode=ReadWrite; Foreign Keys=True; Cache=Default";
 
         private readonly Dictionary<int, Dictionary<int, Dictionary<string, AreaComBucket>>> _memberAreaCommitmentBuckets = [];
         private readonly Dictionary<int, Dictionary<int, Dictionary<string, int>>> _memberDeliveryBuckets = [];