Unfortunately we can't check the 6rd attribute, because it's not
exposed in /sys or anywhere other than netlink... But at least we
can check that networkd brings up an interface that looks right.
It's not entirely impossible to screw something up playing with
kernel modules on a Saturday evening :-) This PR fixes a scenario
where a module has been loaded into the kernel but the module itself
has been removed from the disk.
```
$ lsmod | grep wireg
wireguard 225280 0
ip6_udp_tunnel 16384 1 wireguard
udp_tunnel 16384 1 wireguard
$ modprobe wireguard
modprobe: FATAL: Module wireguard not found in directory /lib/modules/4.18.16-200.fc28.x86_64
$ sudo ./systemd-networkd-tests.py NetworkdNetDevTests.test_wireguard
...
modprobe: FATAL: Module wireguard not found in directory /lib/modules/4.18.16-200.fc28.x86_64
test_wireguard (__main__.NetworkdNetDevTests) ... unexpected success
----------------------------------------------------------------------
Ran 1 test in 5.152s
FAILED (unexpected successes=1)
```
This is a follow-up to https://github.com/systemd/systemd/pull/10625.
After ec6a47044a, networkd starts
soon after removing links used by previous test. That causes
some racy situation of removing links and detecting links by networkd.
