refactor: extract NATS.Server.Auth.Tests project

Move 50 auth/accounts/permissions/JWT/NKey test files from
NATS.Server.Tests into a dedicated NATS.Server.Auth.Tests project.
Update namespaces, replace private GetFreePort/ReadUntilAsync helpers
with TestUtilities calls, replace Task.Delay with TaskCompletionSource
in test doubles, and add InternalsVisibleTo.

690 tests pass.

src/NATS.Server/NATS.Server.csproj

@@ -8,6 +8,7 @@
     <InternalsVisibleTo Include="NATS.Server.Clustering.Tests" />
     <InternalsVisibleTo Include="NATS.Server.Raft.Tests" />
     <InternalsVisibleTo Include="NATS.Server.Monitoring.Tests" />
+    <InternalsVisibleTo Include="NATS.Server.Auth.Tests" />
   </ItemGroup>
   <ItemGroup>
     <FrameworkReference Include="Microsoft.AspNetCore.App" />