fix: increase test timeout from 2 to 5 seconds and add skynet fiber benchmark example

The test runner's per-test timeout was raised to 5 seconds to prevent flaky
timeouts on slower systems. A new example file (example/skynet.wren) was added
demonstrating a million-fiber microbenchmark based on the skynet algorithm,
showcasing Wren's fiber concurrency model. Additionally, the shuffle test's
tolerance threshold was relaxed from 0.2 to 0.21 to reduce false positives
in the random distribution check.

util/test.py

@@ -122,7 +122,7 @@ class Test:
       timed_out[0] = True
       p.kill()
 
-    timer = Timer(2, kill_process, [proc])
+    timer = Timer(5, kill_process, [proc])
 
     try:
       timer.start()