Summary:
It's a common mistake that our tests require foreign extensions (namely evolve
and remotenames) without checking them first.
This diff adds checks to catch these mistakes, adds missing checks, and unifies
our checking logic using `require-ext.sh`, which is aware of `hgext3rd` and
prints skip message.
This affects `arc lint` so hopefully our new testing code would be free of this
kind of mistakes.
Test Plan: `arc lint` would catch errors
Reviewers: #mercurial, ttung, rmcelroy
Reviewed By: rmcelroy
Subscribers: rmcelroy, mjpieters
Differential Revision: https://phabricator.intern.facebook.com/D3550977
Signature: t1:3550977:1468455857:e849dfd9e3cbc446cc6e6c662050ee88a3366e6c
