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/SubList/SubListHighFanoutOptimizationTests.cs

@@ -0,0 +1,22 @@
+using NATS.Server.Subscriptions;
+
+namespace NATS.Server.Core.Tests;
+
+public class SubListHighFanoutOptimizationTests
+{
+    [Fact]
+    public void High_fanout_nodes_enable_packed_list_optimization()
+    {
+        using var sl = new SubList();
+        for (var i = 0; i < 300; i++)
+        {
+            sl.Insert(new Subscription
+            {
+                Subject = "orders.created",
+                Sid = i.ToString(),
+            });
+        }
+
+        sl.HighFanoutNodeCountForTest.ShouldBeGreaterThan(0);
+    }
+}