Add libminijail test executable.

This is useful to test basic Minijail functionality and will be used by
future Autotest tests. The code is currently very similar to
'drop_privs.cpp', but I expect that to change in the future.

Bug: 25368607
Change-Id: I4db3359b07c3b37fa4ac9e24598a5d21623383d5
3 files changed