test: Add stress tests

This patch introduces stress tests, which put load on the daemon to help
sanity check its behaviour under stress.

They are separate from the existing integration tests, which focus on
correctness.

Two tests are included here: a load test, which sends emails repeatedly;
and a connection test which opens as many conections as possible.

test/stress.sh

@@ -0,0 +1,17 @@
+#!/bin/bash
+
+set -e
+. $(dirname ${0})/util/lib.sh
+
+init
+
+FAILED=0
+
+for i in stress-*; do
+	echo $i ...
+	setsid -w $i/run.sh
+	FAILED=$(( $FAILED + $? ))
+	echo
+done
+
+exit $FAILED