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/GoParityRunnerTests.cs

@@ -0,0 +1,21 @@
+namespace NATS.Server.Core.Tests;
+
+public class GoParityRunnerTests
+{
+    [Fact]
+    public void Go_parity_runner_builds_expected_suite_filter()
+    {
+        var cmd = GoParityRunner.BuildCommand();
+        cmd.ShouldContain("go test");
+        cmd.ShouldContain("TestJetStream");
+        cmd.ShouldContain("TestRaft");
+    }
+}
+
+internal static class GoParityRunner
+{
+    public static string BuildCommand()
+    {
+        return "go test -v -run 'TestJetStream|TestJetStreamCluster|TestLongCluster|TestRaft' ./server -count=1 -timeout=180m";
+    }
+}