iotests: Add test for 4G+ compressed qcow2 write

Test what qemu-img check says about an image after one has written
compressed data to an offset above 4 GB.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20191028161841.1198-3-mreitz@redhat.com
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>