Fix misuse of python subprocess module (caused "leaking" processes and garbling the terminal)
- fixed cleanup of Popen objects by pushing spawn logic into test Base and out of test cases
- connect subprocess stdin to PIPE (rather than the parent's STDIN) to fix silent terminal issue

Tested on Linux and Mac OS X



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@175301 91177308-0d34-0410-b5e6-96231b3b80d8