[script] fix starting network manager (#630)

diff --git a/script/_network_manager b/script/_network_manager
index 2d73179..6dff897 100644
--- a/script/_network_manager
+++ b/script/_network_manager
@@ -333,7 +333,7 @@
     if with DNS64; then
         # bind9 provides DNS service
         sudo sed -i 's/^#port=5353/port=0/g' /etc/dnsmasq.conf
-        sudo systemctl stop dnsmasq
+        sudo systemctl restart dnsmasq
     fi
 
     sudo systemctl daemon-reload
@@ -379,9 +379,9 @@
     with NETWORK_MANAGER || return 0
 
     if with DNS64; then
+        sudo systemctl stop dnsmasq
         # revert changes to dnsmasq
         sudo sed -i 's/^port=0/#port=5353/g' /etc/dnsmasq.conf
-        sudo systemctl restart dnsmasq
     fi
 
     if ! have systemctl; then
diff --git a/tests/scripts/check-scripts b/tests/scripts/check-scripts
index 6ac1d95..83f0841 100755
--- a/tests/scripts/check-scripts
+++ b/tests/scripts/check-scripts
@@ -44,6 +44,8 @@
     RELEASE=1 ./script/bootstrap
     ./script/bootstrap
     NAT64=1 ./script/setup
+    # re-run to ensure the script can run successfully multiple times
+    NAT64=1 ./script/setup
     SOCAT_OUTPUT=/tmp/ot-socat
 
     socat -d -d pty,raw,echo=0 pty,raw,echo=0 >/dev/null 2>$SOCAT_OUTPUT &