test: add multi-function stress suite and severe load testing

examples/stress-suite/code/cpu/main.py

@@ -0,0 +1,5 @@
+def main():
+    total = 0
+    for i in range(1, 12000):
+        total += (i * i) % 97
+    return f"cpu-ok:{total}"

examples/stress-suite/code/fast/main.py

@@ -0,0 +1,2 @@
+def main():
+    return "fast-ok"

examples/stress-suite/code/json/main.py

@@ -0,0 +1,2 @@
+def main():
+    return '{"status":"ok","service":"json"}'