TEST-64-UDEV-STORAGE: wait for partition devices being created before calling udevadm...

TEST-64-UDEV-STORAGE: wait for partition devices being created before calling udevadm trigger

For some reasons, kernel or sfdisk once remove the created partitions
and recreated them. And if 'udevadm trigger' triggers devices currently
being removed, the udevd does not receive the triggered events, and the
command stuck.

```
[   33.150452] TEST-64-UDEV-STORAGE.sh[546]: + sfdisk --wipe=always /dev/md/mdmirpar
[   33.478336] systemd-udevd[442]: md127: Device is queued (SEQNUM=2163, ACTION=change)
[   33.480153] kernel:  md127: p1 p2 p3
[   33.483772] systemd-udevd[442]: md127p1: Device is queued (SEQNUM=2164, ACTION=add)
[   33.483914] systemd-udevd[442]: md127p2: Device is queued (SEQNUM=2165, ACTION=add)
[   33.484999] systemd-udevd[442]: md127p3: Device is queued (SEQNUM=2166, ACTION=add)
[   33.485564] systemd-udevd[442]: md127: Received inotify event of watch handle 164.
[   33.503016] TEST-64-UDEV-STORAGE.sh[546]: + SYSTEMD_LOG_LEVEL=debug
[   33.503016] TEST-64-UDEV-STORAGE.sh[546]: + timeout 30 udevadm trigger --settle --parent-match /dev/md/mdmirpar
[   33.485905] systemd-udevd[442]: Successfully forked off '(udev-synth)' as PID 3208.
[   33.486067] systemd-udevd[442]: md127: Removing watch handle 164.
[   33.489035] systemd-udevd[442]: md127p1: Device is queued (SEQNUM=2167, ACTION=remove)
[   33.489048] systemd-udevd[442]: Received inotify event about removal of watch handle 164.
[   33.489507] systemd-udevd[442]: md127p2: Device is queued (SEQNUM=2168, ACTION=remove)
[   33.496298] systemd-udevd[442]: md127p3: Device is queued (SEQNUM=2169, ACTION=remove)
[   33.500628] systemd-udevd[442]: md127: Device is queued (SEQNUM=2170, ACTION=change)
[   33.502355] systemd-udevd[442]: md127p1: Device is queued (SEQNUM=2171, ACTION=add)
[   33.509371] TEST-64-UDEV-STORAGE.sh[3211]: md127: Triggered device with action 'change'.
[   33.509371] TEST-64-UDEV-STORAGE.sh[3211]: md127p1: Triggered device with action 'change'.
[   33.509371] TEST-64-UDEV-STORAGE.sh[3211]: md127p2: Triggered device with action 'change'.
[   33.512532] systemd-udevd[442]: md127: Device is queued (SEQNUM=2172, ACTION=change, UUID=a0b75692-08ad-428a-859b-9ef8772874d7)
[   33.512666] systemd-udevd[442]: md127p1: Device is queued (SEQNUM=2173, ACTION=change, UUID=4cd75a91-aa5b-4678-878c-0420b6c2e1e9)
[   33.512796] systemd-udevd[442]: md127p2: Device is queued (SEQNUM=2174, ACTION=add)
[   33.512910] systemd-udevd[442]: md127p3: Device is queued (SEQNUM=2175, ACTION=add)
[   33.531834] TEST-64-UDEV-STORAGE.sh[3211]: md127: Got uevent without UUID, ignoring: No such file or directory
[   33.553563] TEST-64-UDEV-STORAGE.sh[3211]: md127p1: Got uevent without UUID, ignoring: No such file or directory
[   33.561262] TEST-64-UDEV-STORAGE.sh[3211]: md127p2: Got uevent without UUID, ignoring: No such file or directory
[   33.562468] TEST-64-UDEV-STORAGE.sh[3211]: md127p2: Got uevent without UUID, ignoring: No such file or directory
[   33.563143] TEST-64-UDEV-STORAGE.sh[3211]: md127p3: Got uevent without UUID, ignoring: No such file or directory
[   33.564174] TEST-64-UDEV-STORAGE.sh[3211]: md127p1: Got uevent without UUID, ignoring: No such file or directory
[   33.567614] TEST-64-UDEV-STORAGE.sh[3211]: md127p3: Got uevent without UUID, ignoring: No such file or directory
[   33.597750] TEST-64-UDEV-STORAGE.sh[3211]: md127: Got uevent without UUID, ignoring: No such file or directory
[   33.623522] TEST-64-UDEV-STORAGE.sh[3211]: md127p1: Got uevent without UUID, ignoring: No such file or directory
[   33.676268] TEST-64-UDEV-STORAGE.sh[3211]: md127p3: Got uevent without UUID, ignoring: No such file or directory
[   33.686088] TEST-64-UDEV-STORAGE.sh[3211]: md127p2: Got uevent without UUID, ignoring: No such file or directory
```

Let's wait for partition devices being actually created, and wait for
all queued events being processed. Then, call 'udevadm trigger'.