I have a test suit for my app. As the test suit grew organically, the tests have a lot of repeated code which can be refactored.
However I would like to ensure that the test suite doesn't change with the refactor. How can test that my tests are invariant with the refactor.
(I am using Python+UnitTest), but I guess the answer to this can be language agnostic.
In theory you could write a test for the test, mocking the actualy object under test.But I guess that is just way to much work and not worth it.
So what you are left with are some strategies, that will help, but not make this fail safe.

Work very carefully and slowly. Use the features of you IDEs as much as possible in order to limit the chance of human error.
Work in pairs. A partner looking over your shoulder might just spot the glitch that you missed.
Copy the test, then refactor it. When done introduce errors in the production code to ensure, both tests find the the problem in the same (or equivalent) ways. Only then remove the original test.
The last step can be done by tools, although I don't know the python flavors. The keyword to search for is 'mutation testing'. 

Having said all that, I'm personally satisfied with steps 1+2.
How do i test/refactor my tests?