media: Kconfig: include test_drivers with MEDIA_TEST_SUPPORT

Include test_drivers/Kconfig when MEDIA_TEST_SUPPORT is enabled rather
than MEDIA_PLATFORM_SUPPORT.  Test drivers should not depend on
platform drivers to be enabled.

Reported-by: "kernelci.org bot" <bot@kernelci.org>
Fixes: 4b32216a ("media: split test drivers from platform directory")
Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>