When you write a unit test, you are writing code that expresses intended behavior in a different way than the original implementation. The probability of making exactly corresponding errors (i.e. the implementation is wrong but the test incorrectly checks it and erroneously passes) is lower than the probability of making errors in the original implementation independently of any tests. If your test is incorrect, chances are good that it is wrong in a different way than your implementation, and ideally this will trigger a failure that will lead you to recognize your mistake and fix it.

If you do not feel that first-degree tests offer enough confidence in the correctness of your code, then by all means write tests for the tests. But that many will find this idea absurd demonstrates that the cost/benefit ratio diminishes the more meta you get. (EDIT: Especially since integration and other tests also help contribute to the confidence that the code is correct.)

Alternatively, if your unit test is so similar to the implementation itself that corresponding errors (between the implementation and the test) are likely, then it is probably a poorly written test and of little value.