Call to witness. Testor— witness.

testis: just call yourself to a judge and witness and plea your case!

For *terstis, from  Proto-Italic *tristos, from Proto-Indo-European *tristhâ‚‚s (“a third party standing”, after the two parties to a contract or dispute), from *tréyes (“three”) and *stehâ‚‚- (“to stand”).