Hear any sirens last night? Or notice anyone laying in the grass nearby?
At the price gas is, it would be an expensive prank. I figure someone ran out, and was walking back with a can and passed out or died.
As for testing it, I don't know of a cost effective way.
Most engine harmful things you can put in are soluble in water.
Like sugar.
You could pour about a gallon into a 5 gallon tank and add about 2 gallons of water.
Mix well and decant off the gas.
If there is any sugar in it, it should be in the water.
Keep doing that with fresh water until you have washed all the gas.
Then use it in your lawn mower or something not too expensive.