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