ci: add basic integration test for pmb

Yes, I know this is crude/basic, but others are welcome to improve it later. I think we need some integration testing asap, and this is better than nothing :D

.ci/integration.sh

@@ -7,6 +7,7 @@ if [ "$(id -u)" = 0 ]; then
 fi
 
 pmaports="$(cd "$(dirname "$0")"/..; pwd -P)"
+pmbootstrap="./pmbootstrap.py"
 # Make sure that the work folder format is up to date, and that there are no
 # mounts from aborted test cases (pmbootstrap#1595)
 pmbootstrap work_migrate