test: Add a test for Exim interactions

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.

test/t-exim/hosts

@@ -0,0 +1,2 @@
+srv-chasquid localhost
+srv-exim localhost