testing: Add delay after qemu-nbd -c

testing: Add delay after qemu-nbd -c

Seems that there is a delay before the disk can be used when using newer
QEMU versions (e.g. on Debian trixie). We get errors like these:

  sfdisk: cannot open /dev/nbd0: Inappropriate ioctl for device

or

  mount: /srv/strongswan-testing/build/loop: special device /dev/nbd0p1 does not exist.

A sleep before the next command seems to help.

Also see [1].

[1] https://gitlab.com/qemu-project/qemu/-/issues/1413