I found a bug in SetCommandString() which I plan to fix in a separate patch. I sat down to write a test for the bug and realized there was no SBArgs. So this adds SBArgs so that we can make sure this kind of thing doesn't happen again in the future. Here's an output of my command session verifying that it works as expected.
Python Interactive Interpreter. To exit, type 'quit()', 'exit()' or Ctrl-D.
args = lldb.SBArgs()
args.SetCommandString("This is a test")