fix(challenges): improved tests for adding background/background-image

[jonathan-grah]

Using the background-image property instead of background should still pass the tests.

[RandellDawson]

So are you suggesting the challenge instructions should be updated to explain how to use both background and background-image properties? Maybe the instructions can be changed to reflect how both can be used to change the background image, where background-image can only specify the image and the the background property can specify other properties ( ) in the same line. If there are not already other challenges dealing with the other property value which it can set, the I think there would need to be an additional challenge added to deal with them.

Maybe instead of this lesson teaching about the background property and only specifying the image, the lesson should be about the background-image property and the tests should only pass if background-image property is used. I think this lesson could lead into a next lesson where the background property is discussed in more detail.