Don't do this! PLEASE!
First of all, this is just simply not a good idea. It proves very little, if anything, about functionality of the GFCI. In fact, if you have a GFCI with ground not connected, this might not trip at all, as the electricity won't actually flow through the ground wire (since it is not connected to anything - no complete circuit) and therefore all flow back through neutral until someone gets curious and wiggles the wet extension cord and gets themselves to be part of the circuit! It is actually dangerous, particularly if the GFCI, for some reason, does not trip when it should. Don't do this.
As far as the actual question regarding salt. Salt is NOT a good conductor. Pure water is NOT a good conductor. Water with dissolved salt conducts better than pure water, but it still doesn't conduct all that well. The danger with water and ground-faults is that even if the water only conducts a little bit, it conducts far better than air and conducts enough to kill. You may think of "electric chair" type voltage/current to kill, but a very small current at 120V is enough to kill if it goes in one arm and out the other through your heart.
If you don't trust the built-in tester (and not knowing how it works, that is actually a reasonable consideration), get a plug-in tester. The plug-in testers work by diverting a known amount of current to the ground pin within a safe, insulated, device. FYI, the plug-in testers will (like the crazy extension cord bucket test) fail if there is no ground wire connected to the GFCI, but that does not mean the GFCI won't function properly.
While we're at it, don't do this either...
Do not test your smoke detector by burning some paper, instead of relying on those oh-so-unreliable-just-like-a-GFCI built-in test button. While holding that little piece of paper you may drop it and start a fire. And if it doesn't set off the detector because it didn't produce enough smoke of the right particle size, you'll be tempted to make bigger fires until the bonfire gets out of control... And if you do manage to set off the detector with a small, contained, safe, fire, you will inevitably find out that your wired smoke detector is connected to an alarm control panel that automatically calls the fire department while simultaneously activating alarms through the entire high-rise building. Inevitably resulting in a fine for a false alarm and/or an eviction notice.