When I had to switch from my usual Linux dev box to Mac 10.7 recently (because of Sandy hurricane), I noticed a problem with CURL when I was running my tests through command line.
Basically CURL was just returning output to STDOUT instead of passing it to a variable as it should because of RETURNTRANSFER option in the simpletest class.
After some debugging I figured that the problem was related to the
CURLOPT_COOKIEJAR => $this->cookieFile line. I understood the idea of having clean environment for each test run, but some versions of CURL just don’t like when cookieFile is NULL.
Here is the small patch for Drupal 6:
The problem seems to be fixed in a similar way in Drupal 7 and up according to http://drupal.org/node/1671200.