Tests: Add in-memory database for testing

Tests/HelpersBillingTest.cs

@@ -0,0 +1,30 @@
+using Elwig.Helpers;
+using Microsoft.Data.Sqlite;
+using System.Reflection;
+
+namespace Tests {
+    [TestFixture]
+    public class HelpersBillingTest {
+
+        private SqliteConnection? Connection;
+
+        [OneTimeSetUp]
+        public async Task SetupDatabase() {
+            Connection = await AppDbContext.ConnectAsync();
+            await AppDbContext.ExecuteEmbeddedScript(Connection, Assembly.GetExecutingAssembly(), "Tests.Resources.BillingInsert.sql");
+        }
+
+        [OneTimeTearDown]
+        public async Task TeardownDatabase() {
+            if (Connection == null) return;
+            await AppDbContext.ExecuteEmbeddedScript(Connection, Assembly.GetExecutingAssembly(), "Tests.Resources.BillingDelete.sql");
+            await Connection.DisposeAsync();
+            Connection = null;
+        }
+
+        [Test]
+        public void Test() {
+            // TODO
+        }
+    }
+}