Advocate is a verb that means to support, promote, or champion someone or something. For example, an environmental activist might advocate for a carbon tax. Testify is a legal term referring to giving evidence as a witness in court, like providing evidence in court against a criminal suspect.
Ex: I'm an advocate of animal rights.
Ex: The pandemic has brought many changes. Many people are advocating for a universal healthcare system.
Ex: The mother testified against her own son in court.
Ex: It's often quite stressful for witnesses to testify in court.