test: increase dgram timeout for armv6

test-dgram-broadcast-multi-process.js and
test-dgram-multicast-multi-process.js were failing on Pi 1 because the
test was timing out. Changed static 5000ms timeout to a dynamically
determined timeout based on the processor using
common.platformTimeout().

PR-URL: https://github.com/nodejs/node/pull/2808


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>