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
    ```
    
    (cherry picked from commit 3e624bb13b18b241d00c8d375d5774acde25aa3f)