Add test cases for attaching to a process after fork
- one test case is due to llvm.org/pr16229
- other test case uses a Linux workaround for above by using os.fork() instead of subprocess module

Patch by Andy Kaylor!



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@183340 91177308-0d34-0410-b5e6-96231b3b80d8