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.
It's now an error to sys$mmap() a file as writable if it's currently
mapped executable by anyone else.
It's also an error to sys$execve() a file that's currently mapped
writable by anyone else.
This fixes a race condition vulnerability where one program could make
modifications to an executable while another process was in the kernel,
in the middle of exec'ing the same executable.
Test: Kernel/elf-execve-mmap-race.cpp
