Port the functional server tests from ASOS and OpenIddict 2.x

[kevinchalet]

Both ASOS and OpenIddict have a ton of functional tests that rely on ASP.NET Core's TestServer. As part of the 3.0 effort, we'll need to progressively bring them back:

• ASOS tests: https://github.com/aspnet-contrib/AspNet.Security.OpenIdConnect.Server/tree/dev/test/AspNet.Security.OpenIdConnect.Server.Tests

• OpenIddict tests: https://github.com/openiddict/openiddict-core/tree/2.0.0/test/OpenIddict.Server.Tests/Internal

[kevinchalet]

Most server integration tests have been ported. Only the token creation/validation tests (JWT + DataProtection) need to be added back.