Update the tests for Proxy::Util::CommandTask

The `wget` test was hanging for me - I'm running OS X, and `127.0.0.2` is not a default alias of localhost. I couldn't figure out why the test was using `wget` and neither could the FIXME author. So in this update, use `true` and `false` to test $? return codes 0 and 1, respectively.

That said - Ruby 1.8.7 is heading out the door. If Proxy::Util::CommandTask cannot capture $? on current versions of Ruby, that's problem!
Beware, AFAIK existstatus is not set until you close the file handle.