All 3 Uses

testify
in
Misery, by Stephen King
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The certainty was that Annie's decision to testify in her own behalf at the preliminary hearing had been extremely unwise.†
Chpt 2

testify = provide evidence of something -- especially oral evidence in court
If she had insisted on testifying in her own behalf at her trial, Paul thought, her lawyer probably would have shot her to shut her up.†
Chpt 2 *

testifying = providing evidence of something -- especially oral evidence in court
Her husband never testified for either side.†
Chpt 2

testified = provided evidence of something -- especially oral evidence in court

Definitions:

(1) (testify)
provide evidence of something -- especially to say something under oath in a court of law
(2) (meaning too rare to warrant focus)