refactor: rename remaining tests to NATS.Server.Core.Tests

- Rename tests/NATS.Server.Tests -> tests/NATS.Server.Core.Tests
- Update solution file, InternalsVisibleTo, and csproj references
- Remove JETSTREAM_INTEGRATION_MATRIX and NATS.NKeys from csproj (moved to JetStream.Tests and Auth.Tests)
- Update all namespaces from NATS.Server.Tests.* to NATS.Server.Core.Tests.*
- Replace private GetFreePort/ReadUntilAsync helpers with TestUtilities calls
- Fix stale namespace in Transport.Tests/NetworkingGoParityTests.cs

tests/NATS.Server.Core.Tests/NATS.Server.Core.Tests.csproj

@@ -0,0 +1,28 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="coverlet.collector" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" />
+    <PackageReference Include="NATS.Client.Core" />
+    <PackageReference Include="NSubstitute" />
+    <PackageReference Include="Shouldly" />
+    <PackageReference Include="xunit" />
+    <PackageReference Include="xunit.runner.visualstudio" />
+    <PackageReference Include="Serilog.Sinks.File" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Using Include="Xunit" />
+    <Using Include="Shouldly" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\NATS.Server\NATS.Server.csproj" />
+    <ProjectReference Include="..\NATS.Server.TestUtilities\NATS.Server.TestUtilities.csproj" />
+  </ItemGroup>
+
+</Project>