test: wait for loopback device being actually created

test: wait for loopback device being actually created

It seems there exists a short time period that we cannot see the
loopback device after `losetup` is finished:
```
testsuite-58.sh[367]: ++ losetup -b 1024 -P --show -f /tmp/testsuite-58-sector-1024.img
kernel: loop1: detected capacity change from 0 to 204800
testsuite-58.sh[285]: + LOOP=/dev/loop1
testsuite-58.sh[285]: + systemd-repart --pretty=yes --definitions=/tmp/testsuite-58-sector/ --seed=750b6cd5c4ae4012a15e7be3c29e6a47 --empty=require --dry-run=no /dev/loop1
testsuite-58.sh[368]: Device '/dev/loop1' has no dm-crypt/dm-verity device, no need to look for underlying block device.
testsuite-58.sh[368]: Failed to determine canonical path for '/dev/loop1': No such file or directory
testsuite-58.sh[368]: Failed to open file or determine backing device of /dev/loop1: No such file or directory
```