Add EF Core package references and DbRepositoryTests

Updated `DigitalData.Core.Tests.csproj` to include conditional
package references for Entity Framework Core and its InMemory
provider for net7.0, net8.0, and net9.0.

Added `DbRepositoryTests` class with setup and teardown
methods for managing the lifecycle of a host instance.

DigitalData.Core.Tests/Infrastructure/DbRepositoryTests.cs

@@ -0,0 +1,23 @@
+namespace DigitalData.Core.Tests.Infrastructure;
+
+using Microsoft.Extensions.Hosting;
+
+public class DbRepositoryTests
+{
+    private IHost _host;
+
+    [SetUp]
+    public void Setup()
+    {
+        var builder = Host.CreateApplicationBuilder();
+
+        _host = builder.Build();
+    }
+
+    [TearDown]
+    public void TearDown()
+    {
+        if (_host is IDisposable disposableHost)
+            disposableHost.Dispose();
+    }
+}