test: add a missing udevadm settle to the multipath test case

8a088877ab uncovered a missing `udevadm
settle` in the multipath test case, causing the test to occasionally
fail with:

```
[   41.718617] testsuite-64.sh[806]: + for i in {0..63}
[   41.727917] systemd-udevd[1120]: sdhf: /usr/lib/udev/rules.d/60-persistent-storage.rules:66 Importing properties from results of 'scsi_id --export --whitelisted -d /dev/sdhf'
[   41.728668] testsuite-64.sh[2341]: ++ printf %.4d 1
[   41.733851] systemd-udevd[1415]: sdhc: Adding watch on '/dev/sdhc'
[   41.734477] testsuite-64.sh[806]: + wwid=deaddeadbeef0001
[   41.734477] testsuite-64.sh[806]: + path=/dev/disk/by-id/wwn-0xdeaddeadbeef0001
[   41.746174] systemd-udevd[1409]: sdhb: 'scsi_id --export --whitelisted -d /dev/sdhb'(out) 'ID_MODEL_ENC=QEMU\x20HARDDISK\x20\x20\x20'
[   41.747058] testsuite-64.sh[2342]: ++ readlink -f /dev/disk/by-id/wwn-0xdeaddeadbeef0001
[   41.755512] testsuite-64.sh[806]: + dmpath=/dev/sdi
[   41.755512] testsuite-64.sh[806]: + lsblk /dev/disk/by-id/wwn-0xdeaddeadbeef0001
[   41.766884] systemd-udevd[1402]: sdhd: 'scsi_id --export --whitelisted -d /dev/sdhd'(out) 'ID_VENDOR=QEMU'
[   41.767532] testsuite-64.sh[2343]: NAME MAJ:MIN RM  SIZE RO TYPE MOUNTPOINTS
[   41.767532] testsuite-64.sh[2343]: sdi    8:128  0  512B  0 disk
[   41.779219] systemd-udevd[1400]: dm-33: 'kpartx_id 254 33 mpath-3deaddeadbeef0051'(out) 'DM_WWN=0xdeaddeadbeef0051'
[   41.779902] testsuite-64.sh[806]: + multipath -C /dev/sdi
[   41.785296] systemd-udevd[1414]: dm-31: Device processed (SEQNUM=4839, ACTION=change)
[   41.785925] testsuite-64.sh[2344]: 37.658168 | /dev/sdi is not a dm device
```

test/units/testsuite-64.sh

@@ -132,6 +132,7 @@ EOF
     systemctl start multipathd.service
     systemctl status multipathd.service
     multipath -ll
+    udevadm settle
     ls -l /dev/disk/by-id/
 
     for i in {0..63}; do