tests: add code, public-code and private-code tests

First one is deprecated in favour of the second option.

The latter is newly introduced and annotates the generated symbols
accordingly.

v2: Don't introduce small-public-code.c - reuse small-code.c (Pekka)

Cc: Pekka Paalanen <ppaalanen@gmail.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>