Tip 62: "Test early. Test often. Test Automatically."
PressTest is a memory intensive plugin, particularly while it generates the mock functions.
It attempts to increase the maximum running time during runtime but that can fail depending on your server's settings.
This is something that will be resolved by the first release: till then I recommend using this plugin strictly on your dev setup.
As a hackish way to get around modifying phpUnit, the include path is modified on runtime. If your server doesn't permit changing the include path, then the tests will not run.
Again, this is something that will be resolved by the first release.