Who writes test for their custom assertion methods and such, and can you share that code & tests and walk us through how you did it? Did you apply TDD to writing the test utility methods themselves?
I think this is very important for basic assertions which you'll use again and again. If you can't trust your test framework debugging becomes much, much harder. If you're really stuck on something you'll find yourself wondering about the test code itself.
Even use-once assertions ought to be tested but it's interesting to note that you've already got some protection without doing anything. The code you're writing tests the new (untested) test just as much as the new test tests the code. Obviously that's not perfect but to get a false positive, they both have to fail in step. That can certainly happen but it's not usually a thing which can happen easily.
Obviously there are different levels of acceptable risk. Mistakes in a forum post count aren't so bad but mistakes in a financial app can be catastrophic for a business.
I think use your judgment. If the code is simple it might be OK but if in doubt, test it.