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/t-simple_local/msmtprc

@@ -0,0 +1,21 @@
+account default
+
+host testserver
+port 1025
+
+tls on
+tls_trust_file config/domains/testserver/cert.pem
+
+from user@testserver
+
+auth on
+user user@testserver
+password secretpassword
+
+account baduser : default
+user unknownuser@testserver
+password secretpassword
+
+account badpasswd : default
+user user@testserver
+password badsecretpassword