I think it was "Mark" who stated:


Will the TEST BUTTON on the GFI work if it does not have a ground?

Yes.

If so, how does it work?

Very well[*]

What it does is measure the current leaving the "hot" lead and
compares it with the current returning in the "neutral" lead. If those
are different by more than a few milliamperes, there is a Ground
Fault, that is, current is finding its way to ground through some
other path than "neutral". If that is the case, the GFCI Interrupts
(the "I" part of "GFCI") the Circuit, very, very quickly, so that no
damage is done to whatever caused the Fault (that could be a human
body). The test button simulates a fault.
[*] I had it work once for me. I didn't know it had until the
lamp I had dropped into water wouldn't come on again.

-Don
--
"What do *you* care what other people think?" --Arline Feynman