Make valgrind quiet for child processes

Currently, valgrind reports messages related to child processes and makes the
mspec result REALLY clutter with too many noisy valgrind messages.

So silence the valgrind using --child-silent-after-fork=yes (*1) for child
processes.

As a side effect, valgrind no longer reports messages for child processes. But,
we're temporarily creating child processes only for testing purpose. They
shouldn't be the subject of valgrind. Thus, this shouldn't be a problem.

*1:
  http://valgrind.org/docs/manual/manual-core.html#opt.child-silent-after-fork