WebSocket Stress test using Autobahn TestSuite

This stress test is currently designed to test for full RFC 6455 compliance. However, it's likely we will move to binary-only payloads, at which point we will modify this test accordingly. Currently, this test is disabled by default (via `#define NN_WS_STRESS_SKIP_PERF 1` in `tests/ws_stress.c `), since this test requires Autobahn Testsuite to be installed.