docs/testplan/conf.py: add sphinx path

Without that, extensions from other directories might be probed,
which is not what we want to do.

Such solution was imported from the Linux Kernel conf.py
setting.

Reviewed-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>