media: split test drivers from platform directory

When the first test device was added (vivi.c), there were just
one file. I was too lazy on that time to create a separate
directory just for it, so I kept it together with platform.

Now, we have vivid, vicodec, vim2m and vimc. Also, a new
virtual driver has been prepared to support DVB API.

So, it is time to solve this mess, by placing test stuff
on a separate directory.

It should be noticed that we also have some skeleton drivers
(for V4L and for DVB). For now, we'll keep them separate,
as they're not really test drivers, but instead, just
examples. The DVB frontend ones will likely be part of a new DVB
test driver. By that time, it should make sense to move them
here as well.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>