Port the OpenIddict.Server unit tests from OpenIddict 2.x

[kevinchalet]

The following classes will need to be adapted from 2.x:

• https://github.com/openiddict/openiddict-core/blob/2.0.1/test/OpenIddict.Server.Tests/OpenIddictServerBuilderTests.cs
• https://github.com/openiddict/openiddict-core/blob/2.0.1/test/OpenIddict.Server.Tests/OpenIddictServerExtensionsTests.cs
• https://github.com/openiddict/openiddict-core/blob/2.0.1/test/OpenIddict.Server.Tests/Internal/OpenIddictServerConfigurationTests.cs (for that one, we should remove the dependency on TestServer and make it a proper unit tests class).

[Darthruneis]

@kevinchalet I'm considering taking a look at this one based on our discussion in #956 - can you clarify specifically what you mean when you say 'Port' the tests? My best guess is to fix them/update them after they have broken during changes for 3.0, is that correct?

[kevinchalet]

Thanks @Darthruneis!

My best guess is to fix them/update them after they have broken during changes for 3.0, is that correct?

Yep, exactly. There are also new methods that were added to OpenIddictServerBuilder in 3.0 for which we'll need to add new unit tests after porting the tests class from 2.x (but it can be done in a separate PR).