tests: also use lsmod to check whether modules are available (#10634)

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.