test: update test-child-process-spawn-loop for Python 3

Fixes `SyntaxError: Missing parentheses in call to 'print'.` error when
`python` resolves to Python 3.

Signed-off-by: Richard Lau <riclau@uk.ibm.com>

PR-URL: #34071
Refs: #29298
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Zeyu Yang <himself65@outlook.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: richardlau

test/pummel/test-child-process-spawn-loop.js

@@ -30,7 +30,7 @@ const N = 40;
 let finished = false;
 
 function doSpawn(i) {
-  const child = spawn('python', ['-c', `print ${SIZE} * "C"`]);
+  const child = spawn('python', ['-c', `print(${SIZE} * "C")`]);
   let count = 0;
 
   child.stdout.setEncoding('ascii');