Move shared fixtures and parity utilities to TestUtilities project

- git mv JetStreamApiFixture, JetStreamClusterFixture, LeafFixture,
  Parity utilities, and TestData from NATS.Server.Tests to
  NATS.Server.TestUtilities
- Update namespaces to NATS.Server.TestUtilities (and .Parity sub-ns)
- Make fixture classes public for cross-project access
- Add PollHelper to replace Task.Delay polling with SemaphoreSlim waits
- Refactor all fixture polling loops to use PollHelper
- Add 'using NATS.Server.TestUtilities;' to ~75 consuming test files
- Rename local fixture duplicates (MetaGroupTestFixture,
  LeafProtocolTestFixture) to avoid shadowing shared fixtures
- Remove TestData entry from NATS.Server.Tests.csproj (moved to
  TestUtilities)

tests/NATS.Server.Tests/JetStreamClusterControlApiTests.cs

@@ -1,3 +1,5 @@
+using NATS.Server.TestUtilities;
+
 namespace NATS.Server.Tests;
 
 public class JetStreamClusterControlApiTests
@@ -7,7 +9,7 @@ public class JetStreamClusterControlApiTests
     {
         await using var fx = await JetStreamClusterFixture.StartAsync(nodes: 3);
 
-        var create = await fx.CreateStreamAsync("ORDERS", replicas: 3);
+        var create = await fx.CreateStreamAsync("ORDERS", ["orders.*"], replicas: 3);
         create.Error.ShouldBeNull();
 
         var streamStepdown = await fx.RequestAsync("$JS.API.STREAM.LEADER.STEPDOWN.ORDERS", "{}");