Fix build of tests

Author: sander1095

src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Services/OpenApiDocumentProviderTests.cs

@@ -4,6 +4,7 @@
 using Microsoft.Extensions.ApiDescriptions;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Hosting;
+using Microsoft.OpenApi;
 using Microsoft.OpenApi.Models;
 using Microsoft.OpenApi.Readers;
 using static Microsoft.AspNetCore.OpenApi.Tests.OpenApiOperationGeneratorTests;