I'm not sure I understand the cause of the problem. Does getcwd() fail on Solaris when the path length is higher than PATH_MAX, even if you pass a big enough buffer?
First, your patch makes getcwd() return an error when the path length is longer than PATH_MAX, which it doesn't today. This is a regression and shouldn't probably go in.
Second, the test_posix change is a bit too tolerant. IMO it should check that the error is ERANGE, and that we are under Solaris. Otherwise the error shouldn't happen, should it?
Also, I wonder why py3k uses a simple hard-coded buffer of 1026 bytes (not even PATH_MAX+2). This is even worse than what you are proposing. I'll open a separate issue for it.