This is due to the test using a 50ms delay on hover, and checking "immediately" after generating an "<Enter>" event that it hasn't triggered yet. Note that this isn't actually "immediately": The Tk root's update() is called in between to simulate having a live Tk event loop.
On slow machines this could indeed fail due to the update() call taking a while.
I originally chose 50ms since it seemed like more than enough. Using a longer delay would make the test more robust, but would make testing unnecessarily slow on fast machines.
Perhaps we should look for a more general solution, such as multiplying all test time values by a scale factor depending on a machine's execution speed? That would avoid such future errors in many tests, while keeping the tests faster on fast machines.
Otherwise let's just increase this to 100ms or 200ms.