minijail: add a C test harness
minijail: add a C test harness

Adds a gtest-like, single-file unittest harness for C.  This is groundwork
for adding unittesting code to libminijail and for providing simple
drivers for autotest test cases.

TEST=build the not-yet-committed libminijaiL_unittest and run them
BUG=chromium-os:20917

Change-Id: I822df185399104eb8c1d540cd549a96f0fff061a
Reviewed-on: http://gerrit.chromium.org/gerrit/8370
Reviewed-by: Kees Cook <50cb10d2be062154b17ab4d66a4eab34f1cf1c84@chromium.org>
Reviewed-by: Elly Jones <1693f95ea06ee6d7b2276cb599c3f4f33a951d9c@chromium.org>
Commit-Ready: Will Drewry <72416dde44ea762d19e3a7a555e3b59ce98f8eea@chromium.org>
Tested-by: Will Drewry <72416dde44ea762d19e3a7a555e3b59ce98f8eea@chromium.org>

omegaup/minijail,omegaup/minijail,omegaup/minijail,omegaup/minijail