The West Marin Medical Center is now testing for Covid-19 antibodies—and is aiming to test as many people as possible. Antibody testing, which detects a body’s immune response to the novel coronavirus, is a promising yet unproven tool for public health officials in their pursuit of an end to the pandemic. The center’s testing of patients began on Monday, with Quest Diagnostics running the blood samples in its laboratories and returning the results within two…