Remove test dependency on local config.

The tests depend on both:
 - the systems locale
 - the user sqitch configuration (and maybe the system configuration too)

For example, the add.t test will incorrectly use the users "templates" directory (~/.sqitch/templates), or the revert.t test will assume that no deploy variables are set.

This patch fixes the dependency to the sqitch configuration, by:
 - providing "dummy" user and system configuration for the revert test
 - looking up the user directory in an environment variable before assuming the ~/.sqitch/ directory.

However, the dependency on the systems locale is not fixed. Would it be possible to set the LANG to en_US before each test ?