libevent: disable building test programs

Libevent test programs use fork(), function
that is not available on MMU-less architecture.

This patch prevents compiling libevent test programs
so that library can successfully build on architectures
like blackfin

Fixes:
http://autobuild.buildroot.org/results/2e0/2e0422f1042d0cb00f5bab0dc190930581ed4ef2/



Signed-off-by: Gilles Talis <gilles.talis@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>