Improve TestExitDuringStep to verify thread behaviour during step-over, step-in in addition to step-inst

Patch by Brian Minard!



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@182961 91177308-0d34-0410-b5e6-96231b3b80d8
Find 'ps' command using which() rather than invoking directly
- should address Debian test errors due to not being able to 'ps' directly



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@182965 91177308-0d34-0410-b5e6-96231b3b80d8