AppVeyor: run "vpntest" for ubuntu 18.04

.appveyor.yml

@@ -66,6 +66,7 @@ for:
     - ./configure
   build_script:
     - make package -C tmp
+    - .ci/memory-leak-test.sh
   after_build:
     - .ci/appveyor_afterbuild.sh
   on_failure:

.ci/memory-leak-test.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+
+set -eux
+
+echo -n | ./build/vpntest s | grep -Fq 'NO MEMORY LEAKS'
+echo -n | ./build/vpntest c | grep -Fq 'NO MEMORY LEAKS'
+echo -n | ./build/vpntest b | grep -Fq 'NO MEMORY LEAKS'