I've never found Thunderbird search bad compared to alternatives, as long as I'm not looking to find content inside attachments. Really fast and responsive and being a desktop client without paginated results makes moving and deleting in bulk so much easier. Would love it to be as powerful as Voidtools Everything to get a bit more granular sometimes but otherwise pretty happy with it.

https://www.wordorigins.org/big-list-entries/testify

I'm not sure if this is a pun or whether you have been led to believe that the word testament is derived from the action you state? The additional biblical meaning coming from a confusion of the two meanings of Greek diatheke, which meant both "covenant, dispensation" and "will, testament".