test-50-dissect: do not fail test on cleanup

These paths are read-only mount points. Hence, we cannot remove the
contents.

test/units/testsuite-50.sh

@@ -7,13 +7,17 @@ set -o pipefail
 
 export SYSTEMD_LOG_LEVEL=debug
 
-cleanup()
-{
+cleanup() {(
+    set +ex
+
     if [ -z "${image_dir}" ]; then
         return
     fi
+    umount "${image_dir}/app0"
+    umount "${image_dir}/app1"
+    umount "${image_dir}/app-nodistro"
     rm -rf "${image_dir}"
-}
+)}
 
 udevadm control --log-level=debug