test: bump the post-reboot expect() timeout

as it may take a bit longer on slower machines:

```
[  OK  ] Reached target System Reboot.
Found cgroup2 on /sys/fs/cgroup/, full unified hierarchy
Failed to open watchdog device /dev/watchdog0, ignoring: No such file or directory
binfmt_misc is not mounted, not detaching entries.
Sending SIGTERM to remaining processes...
ERROR:test-shutdown:Timeout exceeded.
<pexpect.pty_spawn.spawn object at 0x7f3d4bcd20b0>
command: /systemd-meson-build/systemd-nspawn
<...snip...>
buffer (last 100 chars): 'mbinfmt_misc is not mounted, not detaching entries.\x1b[0m\r\nSending SIGTERM to remaining processes...\r\n'
before (last 100 chars): 'mbinfmt_misc is not mounted, not detaching entries.\x1b[0m\r\nSending SIGTERM to remaining processes...\r\n'
after: <class 'pexpect.exceptions.TIMEOUT'>
match: None
match_index: None
exitstatus: None
flag_eof: False
pid: 572528
child_fd: 5
closed: False
timeout: 30
delimiter: <class 'pexpect.exceptions.EOF'>
logfile: <_io.TextIOWrapper name='<stdout>' mode='w' encoding='utf-8'>
logfile_read: None
logfile_send: None
maxread: 2000
ignorecase: False
searchwindowsize: None
delaybeforesend: 0.05
delayafterclose: 0.1
delayafterterminate: 0.1
searcher: searcher_re:
    0: re.compile('H login: ')
INFO:test-shutdown:killing child pid 572528
E: nspawn failed with exit code 1
```

test/test-shutdown.py

@@ -73,7 +73,7 @@ def run(args):
 
         logger.info("waiting for reboot")
 
-        console.expect('H login: ', 10)
+        console.expect('H login: ', 30)
         console.sendline('root')
         console.expect('bash.*# ', 10)