Implement IDisposable for derived types for WebApplicationFactory<T> (#7637)

* Implement IDisposable for derived types for WebApplicationFactory<T>

Add a protected Dispose method and a finalizer to WebApplicationFactory<T> for use in classes that derive from it and want to dispose of their own resources.
Resolves #7631.

* Address review feedback

Only dispose of fields if disposing is true.

src/Microsoft.AspNetCore.Mvc.Testing/WebApplicationFactory.cs

@@ -20,6 +20,7 @@ namespace Microsoft.AspNetCore.Mvc.Testing
     /// Typically the Startup or Program classes can be used.</typeparam>
     public class WebApplicationFactory<TEntryPoint> : IDisposable where TEntryPoint : class
     {
+        private bool _disposed;
         private TestServer _server;
         private Action<IWebHostBuilder> _configuration;
         private IList<HttpClient> _clients = new List<HttpClient>();
@@ -54,6 +55,14 @@ namespace Microsoft.AspNetCore.Mvc.Testing
             _configuration = ConfigureWebHost;
         }
 
+        /// <summary>
+        /// Finalizes an instance of the <see cref="WebApplicationFactory{TEntryPoint}"/> class.
+        /// </summary>
+        ~WebApplicationFactory()
+        {
+            Dispose(false);
+        }
+
         /// <summary>
         /// Gets the <see cref="TestServer"/> created by this <see cref="WebApplicationFactory{TEntryPoint}"/>.
         /// </summary>
@@ -327,17 +336,40 @@ namespace Microsoft.AspNetCore.Mvc.Testing
         /// <inheritdoc />
         public void Dispose()
         {
-            foreach (var client in _clients)
+            Dispose(true);
+            GC.SuppressFinalize(this);
+        }
+
+        /// <summary>
+        /// Performs application-defined tasks associated with freeing, releasing, or resetting unmanaged resources.
+        /// </summary>
+        /// <param name="disposing">
+        /// <see langword="true" /> to release both managed and unmanaged resources;
+        /// <see langword="false" /> to release only unmanaged resources.
+        /// </param>
+        protected virtual void Dispose(bool disposing)
+        {
+            if (_disposed)
             {
-                client.Dispose();
+                return;
             }
 
-            foreach (var factory in _derivedFactories)
+            if (disposing)
             {
-                factory.Dispose();
+                foreach (var client in _clients)
+                {
+                    client.Dispose();
+                }
+
+                foreach (var factory in _derivedFactories)
+                {
+                    factory.Dispose();
+                }
+
+                _server?.Dispose();
             }
             
-            _server?.Dispose();
+            _disposed = true;
         }
 
         private class DelegatedWebApplicationFactory : WebApplicationFactory<TEntryPoint>