This patch adds a new test which makes chasquid send and receive email to/from
Exim.
To make it work we need to add two testing flags to the SMTP courier, which is
not ideal but doesn't muddle the code much.
The test is not very portable, and assumes an exim binary is available, but
does not have to be installed in the system. It includes a helper script to
fetch one from the Debian archives.
This patch introduces a new directory, test/, which contains a simple local
end-to-end test which runs a chasquid binary and uses msmtp to send an email,
which is delivered locally.
As it's the first one, it adds a bunch of common infrastructure to simplify
writing these kinds of tests.
More end-to-end tests will follow, and it's expected that the common
infrastructure will also change significantly to accomodate their needs.
