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

[richardlau]

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

Refs: #29298

-bash-4.2$ python --version
Python 3.6.3
-bash-4.2$ ./node test/pummel/test-child-process-spawn-loop.js
stderr:   File "<string>", line 1
    print 1024000 * "C"
                ^
SyntaxError: Missing parentheses in call to 'print'. Did you mean print(1024000 * "C")?

assert.js:103
  throw new AssertionError(obj);
  ^

AssertionError [ERR_ASSERTION]: Expected values to be strictly equal:

0 !== 1024001

    at ChildProcess.<anonymous> (/home/users/riclau/sandbox/github/nodejs/test/pummel/test-child-process-spawn-loop.js:47:12)
    at ChildProcess.emit (events.js:314:20)
    at maybeClose (internal/child_process.js:1051:16)
    at Socket.<anonymous> (internal/child_process.js:442:11)
    at Socket.emit (events.js:314:20)
    at Pipe.<anonymous> (net.js:656:12) {
  generatedMessage: true,
  code: 'ERR_ASSERTION',
  actual: 0,
  expected: 1024001,
  operator: 'strictEqual'
}
-bash-4.2$

This test is in pummel so doesn't get run as part of the regular CI
or default suite of tests (which is probably why this hasn't been
spotted before). It is run as part of the daily/nightly builds but those
seem to be broken due to test-crypto-timing-safe-equal-benchmarks
failures (#34073, cc @Trott).

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/32124/

[richardlau]

Landed in b613933.