Testing unit tests' helper methods

As I am writing tests, some of them have a lot of logic in them. Most of this logic could easily be unit tested, which would provide a higher level of trust in the tests.

I can see a way to do this, which would be to create a class TestHelpers, to put in /classes, and write tests for TestHelpers along with the regular tests.

I could not find any opinion on such a practice on the web, probably because the keywords to the problem are tricky ("tests for tests").

I am wondering whether this sounds like good practice, whether people have already done this, whether there is any advice on that, whether this points to bad design, or something of the sort.

I am running into this while doing characterization tests. I know there are some frameworks for it, but I am writing it on my own, because it's not that complicated, and it gives me more clarity. Also, I can imagine that one can easily run into the same issue with unit tests.

To give an example, at some point I am testing a function that connects to Twitter's API service and retrieves some data. In order to test that the data is correct, I need to test whether it's a json encoded string, whether the structure matches twitter's data structure, whether each value has the correct type, etc. The function that does all these checks with the retrieved data would typically be interesting to test on its own.

Any idea or opinion on this practice ?