With the goal of centralizing all tests in the system, this is a
first step to establish a Tests sub-tree. It will contain all of
the unit tests and test harnesses for the various components in the
system.
This encourages callers to strongly reference file descriptions while
working with them.
This fixes a use-after-free issue where one thread would close() an
open fd while another thread was blocked on it becoming readable.
Test: Kernel/uaf-close-while-blocked-in-read.cpp
