I don't think the home outlet/GFCI tester can test the galvanic isolator.
The GFCI monitors and cuts out the leaks between hot or neutral and ground wires. This is what protects you or swimmers in water.
The galvanic isolator or isolating transformer separates shore ground from boat ground and cuts out stray current coming from a faulty shore installation.
Two different things.
FWIW, there are various testing method described if you google for it.
I have not tried any of them ... one more thing to do this spring, ugh ...