test: s/assert.fail/common.fail as appropriate

Many tests use assert.fail(null, null, msg) where it would be
simpler to use common.fail(msg). This is largely because
common.fail() is fairly new. This commit makes the replacement
when applicable.

PR-URL: https://github.com/nodejs/node/pull/7735


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>