test: Add a new local end-to-end test

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.

test/util/test-mda

@@ -0,0 +1,14 @@
+#!/bin/bash
+
+set -e
+
+mkdir -p .mail
+
+# TODO: use flock to lock the file, to prevent atomic writes.
+echo "From ${1}" >> .mail/.tmp-${1}
+cat >> .mail/.tmp-${1}
+X=$?
+if [ -e .mail/.tmp-${1} ]; then
+	mv .mail/.tmp-${1} .mail/${1}
+fi
+exit $X