selftests: uevent filtering

Recent discussions around uevent filtering (cf. net-next commit [1], [2],
and [3] and discussions in [4], [5], and [6]) have shown that the semantics
around uevent filtering where not well understood.
Now that we have settled - at least for the moment - how uevent filtering
should look like let's add some selftests to ensure we don't regress
anything in the future.
Note, the semantics of uevent filtering are described in detail in my
commit message to [2] so I won't repeat them here.

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=90d52d4fd82007005125d9a8d2d560a1ca059b9d
[2]: https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=a3498436b3a0f8ec289e6847e1de40b4123e1639
[3]: https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=26045a7b14bc7a5455e411d820110f66557d6589
[4]: https://lkml.org/lkml/2018/4/4/739
[5]: https://lkml.org/lkml/2018/4/26/767
[6]: https://lkml.org/lkml/2018/4/26/738

Signed-off-by: Christian Brauner <christian@brauner.io>
Signed-off-by: David S. Miller <davem@davemloft.net>