Add TestThreadAPI.py file to house the Python SBThread API test cases.

Currently it has only test cases for SBThread.GetStopDescription() API.

Also modified lldb.swig to add typemap for (char *dst, size_t dst_len)
which occurs for SBThread::GetStopDescription() C++ API.  For Python
scripting:

    # Due to the typemap magic (see lldb.swig), we pass in an (int)length to GetStopDescription
    # and expect to get a Python string as the result object!
    # The 100 is just an arbitrary number specifying the buffer size.
    stop_description = thread.GetStopDescription(100)

llvm-svn: 127173