test: Rename test directories to use an explicit order

For convenience, rename the directories so the order is explicit in the name.
Make the simple test run first, to save time if it's not working.

test/t-02-exim/get-exim4-debian.sh

@@ -0,0 +1,27 @@
+#!/bin/bash
+#
+# This script downloads the exim4 binary from Debian's package.
+# It assumes "apt" is functional, which means it's not very portable, but
+# given the nature of these tests that's acceptable for now.
+
+set -e
+. $(dirname ${0})/../util/lib.sh
+
+init
+
+# Download and extract the package in .exim-bin
+apt download exim4-daemon-light
+dpkg -x exim4-daemon-light_*.deb $PWD/.exim-bin/
+
+# Create a symlink to .exim4, which is the directory we will use to store
+# configuration, spool, etc.
+# The configuration template will look for it here.
+mkdir -p .exim4
+ln -sf $PWD/.exim-bin/usr/sbin/exim4 .exim4/
+
+# Remove the setuid bit, if there is one - we don't need it and may cause
+# confusion and/or security troubles.
+chmod -s .exim-bin/usr/sbin/exim4
+
+success
+