test: fix pummel/test-exec

Fix test/pummel/test-exec.js which broke as a result of
e47f972
(nodejs#24951).

(Until very recently, pummel tests were not run at all in CI and
currently only run nightly on master.)

test/pummel/test-exec.js

@@ -122,7 +122,7 @@ exec('python -c "print 200000*\'C\'"', { maxBuffer: 1000 },
      function(err, stdout, stderr) {
        assert.ok(err);
        assert.ok(/maxBuffer/.test(err.message));
-       assert.strictEqual(stdout, '');
+       assert.strictEqual(stdout, 'C'.repeat(1000));
        assert.strictEqual(stderr, '');
      });