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/basic.conf

@@ -0,0 +1,19 @@
+port: 4222
+host: "0.0.0.0"
+server_name: "test-server"
+max_payload: 2mb
+max_connections: 1000
+debug: true
+trace: false
+logtime: true
+logtime_utc: false
+ping_interval: "30s"
+ping_max: 3
+write_deadline: "5s"
+max_subs: 100
+max_sub_tokens: 16
+max_control_line: 2048
+max_pending: 32mb
+lame_duck_duration: "60s"
+lame_duck_grace_period: "5s"
+http_port: 8222