test: reorder the machinectl signal tests

test: reorder the machinectl signal tests

So the multiple-machines tests run last to avoid a race where the
checked signal would get ignored while the stub init is still processing
the previous signals:

[   17.380417] testsuite-13.sh[376]: + machinectl reboot long-running long-running long-running
[   17.389888] systemd-nspawn[495]: ++ touch /reboot
[   17.390904] testsuite-13.sh[376]: + rm -f /var/lib/machines/long-running/trap
[   17.393937] testsuite-13.sh[376]: + machinectl kill --signal=SIGTRAP --kill-whom=leader long-running
[   17.408905] testsuite-13.sh[376]: + timeout 10 bash -c 'while ! test -e /var/lib/machines/long-running/trap; do sleep .5; done'
[   27.413210] testsuite-13.sh[376]: + at_exit
...
[   27.993376] testsuite-13.sh[373]: + echo 'Subtest /usr/lib/systemd/tests/testdata/units/testsuite-13.machinectl.sh failed'