tests/Makefile.inc: Fix config.h inclusion

Unlike in coreboot, in depthcharge we need to explicitly include
config.h in Makefiles. Therefore, instead of adding test-specific config
directory to search path, include the config file directly.

BUG=b:172343578
TEST=make unit-tests
BRANCH=none

Change-Id: Ica2d1d470f9160f5dc072ec4e5d50e0194eb64fa
Signed-off-by: Yu-Ping Wu <yupingso@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/depthcharge/+/3039500


Reviewed-by: Julius Werner <jwerner@chromium.org>