support/testing: use virtio-rng-pci to test syslog-ng

support/testing: use virtio-rng-pci to test syslog-ng

Recent versions of syslog-ng need some entropy on startup.
So use VirtIORNG to provide it. In order to accomplish this:
 - build the kernel containing the driver;
 - pass '-device virtio-rng-pci' to qemu.

Use the same kernel version and kernel config as qemu_arm_versatile.
It already has PCI enabled but it does not have HW_RANDOM_VIRTIO, so add
a defconfig fragment to enable the drivers.

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/259856394

At the same time, fix a typo (missing '#') that resulted in the
generation of root.tar. This file is not used in the test.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>