test: increase bufsize in child process write test

test-child-process-stdio-big-write-end was failing on ubuntu1604-arm64
because the while loop that was supposed to fill up the buffer ended up
being an infinite loop.

This increases the size of the writes in the loop by 1K until the buffer
fills up.

PR-URL: https://github.com/nodejs/node/pull/13626
Fixes: https://github.com/nodejs/node/issues/13603


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>