Fix syntax error in nested process test

The grandchild_script was using unescaped triple quotes inside an
f-string, causing a syntax error. Fixed by escaping the triple quotes
and adjusting indentation.

Author: felixweinberger

tests/client/test_stdio.py

@@ -490,12 +490,12 @@ async def test_stdio_client_nested_process_tree():
             import time
 
             # Grandchild just writes to file
-            grandchild_script = """import time
-            with open({repr(grandchild_file)}, 'a') as f:
-                while True:
-                    f.write(f"gc {time.time()}")
-                    f.flush()
-                    time.sleep(0.1)"""
+            grandchild_script = \"\"\"import time
+with open({repr(grandchild_file)}, 'a') as f:
+    while True:
+        f.write(f"gc {{time.time()}}")
+        f.flush()
+        time.sleep(0.1)\"\"\"
 
             # Spawn grandchild
             subprocess.Popen([sys.executable, '-c', grandchild_script])