libmemcached: disable tests

Disable test libraries and programs, even though it needs to autoreconf
it's a good tradeoff since it cuts the build time by half, and avoids
build issues with extra dependencies that aren't properly checked for.
Fixes:
http://autobuild.buildroot.net/results/739/739ae9b2f28486dec8900aa584145d2d38f44331/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>