Add a test case test_image_search_paths() to test lldb's ability to do image search paths

substitutions in order to achieve file mappings.

Modify CommandObjectTarget.cpp to properly set the status of the return object to make
scripting like this:

    self.runCmd("target image-search-paths add %s %s" % (os.getcwd(), new_dir))

works.

llvm-svn: 124762