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.
Don't refactor the test suite.
The purpose of refactoring is to make it easier to maintain the code, not to satisfy some abstract criterion of "code niceness". Test code doesn't need to be nice, it doesn't need to avoid repetition, but it does need to be thorough. Once you have a test that is valid (i.e. it really does test necessary conditions on the code under test), you should never remove it or change it, so test code doesn't need to be easy to maintain en masse.
If you like, you can rewrite the existing tests to be nice, and run the new tests in addition to the old ones. This guarantees that the new combined test suite catches all the errors that the old one did (and maybe some more, as you expand the new code in future).
There are two ways that a test can be deemed invalid -- you realise that it's wrong (i.e. it sometimes fails falsely for correct code under test), or else the interface under test has changed (to remove the API tested, or to permit behaviour that previously was a test failure). In that case you can remove a test from the suite. If you realise that a whole bunch of tests are wrong (because they contain duplicated code that is wrong), then you can remove them all and replace them with a refactored and corrected version. You don't remove tests just because you don't like the style of their source.
To answer your specific question: to test that your new test code is equivalent to the old code, you would have to ensure (a) all the new tests pass on your currently-correct-as-far-as-you-known code base, which is easy, but also (b) the new tests detect all the errors that the old tests detect, which is usually not possible because you don't have on hand a suite of faulty implementations of the code under test.
How do i test/refactor my tests?