Fixes:
$ cd test/TEST-07-ISSUE-1981/
$ sudo make clean setup run
...
timeout: failed to run command ‘systemd-nspawn’: No such file or directory
...
TEST RUN: https://github.com/systemd/systemd/issues/1981 [FAILED]
Makefile:10: recipe for target 'run' failed
make: *** [run] Error 1