tests: fix race condition between mkfs and udev

Just after LVM volume is created, udev tries to analyze it. This
prevents mkfs from creating filesystem there.

qubes/tests/storage_lvm.py

@@ -1009,6 +1009,7 @@ class TC_02_StorageHelpers(ThinPoolBase):
                 'test-file-pool'], stdout=subprocess.DEVNULL)
         self.thin_dev = '/dev/{}/test-file-pool'.format(
             DEFAULT_LVM_POOL.split('/')[0])
+        subprocess.check_call(['udevadm', 'settle'])
         subprocess.check_call(
             ['sudo', 'mkfs.ext4', '-q', self.thin_dev])
         subprocess.check_call(['sudo', 'mount', self.thin_dev,