Improve bash quoting, and other similar best practices

This patch updates the shell scripts with some of the common best
practices, which should make them more resilient to unusual failures and
unexpected environments (in particular, directories with spaces).

Most of these were identified by shellcheck.

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

@@ -5,19 +5,19 @@
 # given the nature of these tests that's acceptable for now.
 
 set -e
-. $(dirname ${0})/../util/lib.sh
+. "$(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 $PWD/.exim-bin/
+dpkg -x exim4-daemon-light_*.deb "$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 $PWD/.exim-bin/usr/sbin/exim4 .exim4/
+ln -sf "$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.

test/t-02-exim/run.sh

@@ -23,7 +23,7 @@
 #   chasquid will receive the email from exim, and deliver it locally.
 
 set -e
-. $(dirname ${0})/../util/lib.sh
+. "$(dirname "$0")/../util/lib.sh"
 
 init
 check_hostaliases