Testimony began Tuesday in the murder trial of Matthew Farwell, a former Stoughton police officer accused of killing Sandra Birchmore, who was 23 and pregnant when she died. Prosecutors allege Farwell killed Birchmore and staged her death as a suicide to conceal a sexual relationship that began when she was 15. They say surveillance footage shows Farwell entering Birchmore’s apartment wearing a mask to cover his face. Farwell’s defense has argued that Birchmore died by suicide. In opening statements Friday, defense attorneys said the mask was not intended to hide a crime, but was connected to concealing his infidelity from his wife. One of the first witnesses was expected to be a coworker of Birchmore’s who contacted police after she failed to show up for work. The jury will weigh the evidence and the competing accounts as the trial proceeds.