Moved libprocess tests into a subdirectory.

From: Ben Mahler <benjamin.mahler@gmail.com>
Review: https://reviews.apache.org/r/8792

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1440851 13f79535-47bb-0310-9956-ffa450edef68
Split apart libprocess tests into separate, smaller, files.

From: Ben Mahler <benjamin.mahler@gmail.com>
Review: https://reviews.apache.org/r/8793

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1440852 13f79535-47bb-0310-9956-ffa450edef68