Autotest suites need /tmp mounted exec.
Autotest suites need /tmp mounted exec.

Some suites need /tmp to be mounted exec.  This change adds exec back to /tmp when mod_for_test is set.

Change-Id: Ia15deba354240cfa036c5bfeecd9ac6fb532edb7

BUG=7286
TEST=Make change, run BVT
TBR=wad@chromium.org

joshfire/chromiumos-platform-init