Multiple sandboxes test: Use NaClExit() instead of an implicit exit()
Multiple sandboxes test: Use NaClExit() instead of an implicit exit()

This is on the hunch that the non-deterministic failures in this test
that happen during NaClSysCommonThreadSuicide()/NaClAppThreadDtor()
(called in a detached/unjoined thread) are caused by process-global
destructors which are called by exit().

BUG=http://code.google.com/p/nativeclient/issues/detail?id=1485
TEST=run_multidomain_test

Review URL: http://codereview.chromium.org/7552025

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@6379 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

nacl-webkit/native_client,sbc100/native_client,sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,sbc100/native_client,nacl-webkit/native_client