refactor: extract NATS.Server.Clustering.Tests project

Move 29 clustering/routing test files from NATS.Server.Tests to a
dedicated NATS.Server.Clustering.Tests project. Update namespaces,
replace private GetFreePort/ReadUntilAsync helpers with TestUtilities
calls, and extract TestServerFactory/ClusterTestServer to TestUtilities
to fix cross-project reference from JetStreamStartupTests.

tests/NATS.Server.Clustering.Tests/RoutePoolTests.cs

@@ -0,0 +1,11 @@
+namespace NATS.Server.Clustering.Tests;
+
+public class RoutePoolTests
+{
+    [Fact]
+    public async Task Route_manager_establishes_default_pool_of_three_links_per_peer()
+    {
+        await using var fx = await RouteFixture.StartTwoNodeClusterAsync();
+        (await fx.ServerARouteLinkCountToServerBAsync()).ShouldBe(3);
+    }
+}