Can the EHT M87 results be used to test general relativity?

No. All theoretical predictions for the observational appearance of an accreting supermassive black hole, as measured interferometrically by a sparse Earth-sized array at current observation frequencies, are sensitive to many untested assumptions about accretion flow and emission physics. There is no way to distinguish a violation of general relativity (GR) from the much more likely scenario that the relevant "gastrophysical" assumptions simply do not hold. Tests of GR will become possible with longer interferometric baselines (likely requiring a space mission) that reach the resolution where astrophysics-independent predictions of the theory become observable.