Fix 'platform shell' command for Linux host and remote lldb-platform connections
- add default timeout of 10s (unil qPlatform_RunCommand supports timeout packets and CommandObjectPlatform is updated to read a timeout flag/setting)
- add a few tests for platform shell



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@189405 91177308-0d34-0410-b5e6-96231b3b80d8