media: Makefile: fix test drivers compilation

test_drivers/ folder is not being added by media Makefile, so it is not
being compiled.

Add test_drivers/ folder in Makefile folder's list.

Fixes: 4b32216a ("media: split test drivers from platform directory")
Signed-off-by: Helen Koike <helen.koike@collabora.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>