test: fix flaky test-child-process-spawnsync-input

Move portion of `test-child-process-spawnsync-input.js` (that has been
flaky on CentOS in CI) to its own file. This allows us to more easily
eliminate the cause of the flakiness without affecting other unrelated
portions of the test.

Fixes: https://github.com/nodejs/node/issues/3863
PR-URL: https://github.com/nodejs/node/pull/3889


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>