Are "weather sealed" gear tested for how good their "weather sealing" is?

AFAIK, cameras aren't tested and rated against some independent standard -- if they were, manufacturers would certainly advertise that fact. Equipment is usually described as being weather sealed if it is, and weather sealing just isn't mentioned if it's not. Equipment reviews typically make an attempt to describe the degree and/or locations of weather sealing, for example:

The high build quality is very much to the normal Canon L-grade standard, or even a touch above it with magnesium alloy for the outer barrel (the extending front section is engineering-grade plastic) and now with a full complement of weather-resistant seals.

There's a general understanding that weather sealed doesn't mean waterproof or even water resistant. If you're out in rain, snow, or dust with your weather sealed camera and lens, your camera will probably survive just fine. If you drop your camera in a pond, not so much.

Given how vague they are, I don't know how beneficial it is?

Think of it like the airbags in your car: you don't know exactly how much they'll protect you, but it's still good to know that they're there.

Let common sense apply -- if you get caught in the rain with an unsealed camera, you should probably tuck it inside your jacket and find some shelter. If you get caught in the rain with a sealed camera, it's fine to keep shooting, but you don't necessarily want to expose it to a torrential downpour with driving wind.