tests: add test for https://github.com/systemd/systemd/issues/2637

tests: add test for https://github.com/systemd/systemd/issues/2637

+ perl -e 'exit(!(qq{0} eq qq{\x25U}))'
exec-spec-interpolation.service: Main process exited, code=exited, status=1/FAILURE
exec-spec-interpolation.service: Unit entered failed state.
exec-spec-interpolation.service: Failed with result 'exit-code'.
        PID: 11270
        Start Timestamp: Wed 2016-02-17 22:21:31 UTC
        Exit Timestamp: Wed 2016-02-17 22:21:31 UTC
        Exit Code: exited
        Exit Status: 1
Assertion 'service->main_exec_status.status == status_expected' failed at src/test/test-execute.c:65, function check(). Aborting.