package/yaml-cpp: disable testing and parse tools

The gmock testing framework uses fork() and therefor yaml-cpp fails to build
for a MMU-less targets. Since we are only interested in building the yaml-cpp
library we disable the testing and parse tools (command line tools).

Fixes:
http://autobuild.buildroot.net/results/2ce/2ce3c6d0140696cdf73f2ad1499b779c83ad8f6e/



Cc: Sebastien Bourdelin <sebastien.bourdelin@savoirfairelinux.com>
Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>