Add comprehensive test suite for loadtest server

[jrepp]

Summary

• 36 tests covering HTTP API, WebSocket, and integration scenarios
• All tests pass with no flakiness detected
• Integration tests use real Redis and NATS backends
• Validated across 3 consecutive test runs

Test Coverage

Unit Tests (27 tests)

Server API Tests (15): server_test.go

• Test creation and initialization
• HTTP endpoint handlers (start/stop/status/list)
• JSON serialization and error responses
• CORS middleware with OPTIONS support
• Graceful shutdown with nil safety
• Empty array handling

Executor Tests (12): executor_test.go

• Test lifecycle management
• Metrics collection and aggregation
• WebSocket client management
• Broadcasting with dead client removal
• Context cancellation handling

Integration Tests (9 tests)

End-to-end scenarios: integration_test.go

• Start/stop test lifecycle with real backends
• WebSocket streaming with live metrics
• Multiple concurrent WebSocket clients (3 simultaneous)
• Concurrent tests with unique IDs (microsecond precision)
• Server shutdown with active tests
• Dashboard HTTP access
• CORS headers validation
• WebSocket reconnection handling
• Error scenarios (invalid JSON, non-existent tests)

Bugs Fixed During Testing

1. Test ID collision: Changed format from second to microsecond precision
2. Nil pointer panic: Added nil check in Shutdown() method
3. Empty array JSON: Initialize slices with make([]T, 0) instead of var
4. CORS 405 errors: Added OPTIONS method to all routes
5. Broadcast race condition: Changed RLock to Lock for client slice modification

Test Results

# Pass 1
PASS - 36 tests, 11.407s

# Pass 2  
PASS - 36 tests, 11.219s

# Pass 3
PASS - 36 tests, 11.246s

Flakiness: None detected across 3 consecutive runs

Test plan

• Run with go test -v ./...
• Integration tests require Redis (port 6379) and NATS (port 4222)
• Use -short flag to skip integration tests
• All tests pass consistently

Stack

• PR 1: Metrics refactor (Refactor: Move MetricsCollector to server package #212)
• PR 2: Server implementation (Add loadtest HTTP server with WebSocket support #213)
• PR 3 (this PR): Test suite (depends on Add loadtest HTTP server with WebSocket support #213)

[mergify]

This PR has been inactive for 14 days. Please update it or close it if it's no longer needed.