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

@@ -3,7 +3,7 @@
 set -x
 
 if [ "$(id -u)" = 0 ]; then
-	exec su "${TESTUSER:-pmos}" -c "sh -e $0"
+	exec su "${TESTUSER:-build}" -c "sh -e $0"
 fi
 
 pmaports="$(cd "$(dirname "$0")"/..; pwd -P)"