test: Skip exim test if the binary is not found

The exim test requires a bit more preparation than the rest, so skip it if the
binary is not found or not working properly.

test/t-02-exim/run.sh

@@ -27,6 +27,11 @@ set -e
 
 init
 
+if ! .exim4/exim4 --version > /dev/null; then
+	skip "(exim4 binary at .exim4/exim4 is not functional)"
+	exit 0
+fi
+
 # Create a temporary directory for exim4 to use, and generate the exim4
 # config based on the template.
 mkdir -p .exim4

test/util/lib.sh

@@ -58,6 +58,10 @@ function success() {
 	echo "SUCCESS"
 }
 
+function skip() {
+	echo "SKIPPED" $*
+}
+
 # Wait until there's something listening on the given port.
 function wait_until_ready() {
 	PORT=$1