Bad idea? tollay new at this

Greg,

Your test won't provide enough water pressure to test properly.

It's probably a decent DWR, but not waterproof, IMO. However, it could be PU coated….waterproof!

Others can give a better testing recommendation than I. However, try setting up a bigger section and spray a hose at it from various distances & water pressures. Lay a towel underneath to see if there's leakage.

Shiny coating?
That is likely to be acrylic. It is not really all that waterproof. An acrylic coating is very cheap and is commonly used to help the fabric keep its shape.

However … Have a Go!
Provided you pitch the tarp with a bit of a slope on the roof and you seam-seal the ridge somehow, it will probably work moderately well for a couple of years.
Have a Go!

Cheers