Skip Mercurial tests if hg is not present
Summary: I don't have hg yet on my new laptop; we should just skip tests if the user is missing binaries. Add a convenience method to do this.
Test Plan: Got clean arc unit --everything with no hg installed.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Differential Revision: https://secure.phabricator.com/D11051