Fix a race condition where you could set the selected thread & target in the

Fix a race condition where you could set the selected thread & target in the
CommandInterpreter's execution context AFTER the process had started running
and before it initially stopped.  Also fixed one test case that was implicitly
using this (and an abuse of the async mode) to accidentally succeed.

<rdar://problem/16814726>

llvm-svn: 226528