test: allow for slow hosts in spawnSync() test

test-child-process-spawnsync-timeout failed from time to time on
Raspberry Pi devices. Use common.platformTimeout() to allow a little
more time to run on resource-constrained hosts.

PR-URL: https://github.com/nodejs/node/pull/10998


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>