Execute configurable script before and after running backstop

bin/backstop

@@ -2,7 +2,9 @@
 
 export $(cat .env | xargs)
 cd tests/backstop
+./backstop-script-before
 docker run --rm -t -v $(php /usr/local/bin/volume.php) -w $PWD --network=traefik-public backstopjs/backstopjs "$@"
+./backstop-script-after
 EC=$?
 
 exit $EC