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/config/chasquid.conf

@@ -0,0 +1,5 @@
+address: ":1025"
+monitoring_address: ":1099"
+
+mail_delivery_agent_bin: "test-mda"
+mail_delivery_agent_args: "%user%@%domain%"

test/t-exim/config/domains/srv-chasquid/users

@@ -0,0 +1,2 @@
+#chasquid-userdb-v1
+user SCRYPT@n:14,r:8,p:1,l:32,r00XqNmRkV505R2X6KT8+Q== aAiBBIVNNzmDXwxLLdJezFuxGtc2/wcHsy3FiOMAH4c=

test/t-exim/config/exim4.in

@@ -0,0 +1,62 @@
+CONFDIR = ${EXIMDIR}
+
+spool_directory = CONFDIR/spool
+exim_path = CONFDIR/exim4
+
+# No need to keep anything on the environment.
+# This is the default, but exim emits a warning if it's not set
+# (https://www.exim.org/static/doc/CVE-2016-1531.txt).
+keep_environment = 
+
+# Disable TLS for now.
+tls_advertise_hosts =
+
+# Run as the current user.
+# TODO: obviously do not hard-code it!
+exim_group = ${USER}
+exim_user = ${USER}
+
+# Listen on a non-privileged port.
+daemon_smtp_port = 2025
+
+# ACLs to let anyone send mail (for testing, obviously).
+acl_smtp_rcpt = acl_check_rcpt
+acl_smtp_data = acl_check_data
+
+begin acl
+acl_check_rcpt:
+  accept
+
+acl_check_data:
+  accept
+
+
+# Forward all incoming email to chasquid (running on :1025 in this test).
+begin routers
+
+rewritedst:
+  driver = redirect
+  data = someone@srv-chasquid
+
+forwardall:
+  driver = accept
+  transport = tochasquid
+
+begin transports
+
+tochasquid:
+  driver = smtp
+
+  # exim4 will by default detect and special-case deliveries to localhost;
+  # this avoids that behaviour and tells it to go ahead anyway.
+  allow_localhost
+  hosts_override
+
+  # chasquid will be listening on localhost:1025
+  hosts = localhost
+  port = 1025
+
+  # Add headers to help debug failures.
+  delivery_date_add
+  envelope_to_add
+  return_path_add