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.TestUtilities/TestData/tls.conf

@@ -0,0 +1,12 @@
+tls {
+  cert_file: "/path/to/cert.pem"
+  key_file: "/path/to/key.pem"
+  ca_file: "/path/to/ca.pem"
+  verify: true
+  verify_and_map: true
+  timeout: 3
+  connection_rate_limit: 100
+  pinned_certs: ["abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789"]
+  handshake_first: true
+}
+allow_non_tls: false