cosmetic(ish): banish double bracket bashism

test/config.sh

@@ -123,11 +123,11 @@ assert_dns_record() {
     shift 3
     exp_ips_regex=$(echo "$@" | sed -r 's/ /\\\|/g')
 
-    [[ -z "$DEBUG" ]] || greyly echo "Checking whether $name exists at $host:$container"
+    [ -z "$DEBUG" ] || greyly echo "Checking whether $name exists at $host:$container"
     got_ip=$(exec_on $host $container getent hosts $name)
     assert_raises "echo '$got_ip' | grep -q '$exp_ips_regex'"
 
-    [[ -z "$DEBUG" ]] || greyly echo "Checking whether the IPs '$@' exists at $host:$container"
+    [ -z "$DEBUG" ] || greyly echo "Checking whether the IPs '$@' exists at $host:$container"
     for ip in "$@" ; do
         assert "exec_on $host $container getent hosts $ip | tr -s ' '" "$ip $name"
     done
@@ -139,7 +139,7 @@ assert_no_dns_record() {
     container=$2
     name=$3
 
-    [[ -z "$DEBUG" ]] || greyly echo "Checking if '$name' does not exist at $host:$container"
+    [ -z "$DEBUG" ] || greyly echo "Checking if '$name' does not exist at $host:$container"
     assert_raises "exec_on $host $container getent hosts $name" 2
 }