Support Mercurial in test_connection.php

Summary: Add Mercurial support to this script.

Test Plan: Connected to public and private Bitbucket mercurial repos,
successfully (with valid credentials) and unsuccessfully (without valid
credentials).

Reviewers: Makinde, jungejason, nh, tuomaspelkonen, aran

Reviewed By: jungejason

CC: aran, jungejason

Differential Revision: 938