One way you could do it better is to adopt Twisted's test-docstring policy and actually explain in narrative form what the point of these tests is ;).  As I read this, I can see that someone thought that 1 and 4 were bad numbers and 65537 is a good number.  Maybe not everyone knows about the innumerable virtues of 65537?  Maybe it's not clear why 511 and 256 are "too small" but 2048 is "just right"?  Why not 4096?  Why not 8192?  At least use some constants defined in the tests with descriptive names.

Also, these tests shouldn't be as close to the same as they are.  `ValueError` is insufficiently precise.  What does the `ValueError` say?  Does the programmer reading the traceback learn something about what they did wrong, or is it just an opaque error that is the same regardless of which parameter they screwed up and how?  ValueError, being a built-in exception, could also be raised by accident in the middle of a backend's implementation rather than correctly catching the invalid parameters.  (I'd personally define a new exception type rather than using built-in ones; there's no particular reason that `except ValueError` being polymorphic to bad key parameters is useful, as far as I can tell.)