Translation:Why is that test difficult?
Expanation of んです, んだ, のです, のだ http://www.japanese-language.aiyori.org/article10.html
The んだ or んです form basically means "if you know what I mean." Basically, it indicates that the speaker is implying something. Examples:
Phrase: 日本語はむずかしいんです。Japanese is difficult, if you know what I'm implying. こまっています。しゅくだいをわすれたんです。I am in trouble. I forgot my homework. (In this case, implying that that's the reason you're in trouble.)
Conversation: 「外に行っています」I'm going outside. 「天気は雨んです」It's rainy outside (if you know what I'm implying).
In my experience of spending a few years in Japan and living with Japanese the "n" sound or "no" is used for emphasis like a small exclamation point. Someone might say, ii desu ka? and one answer ii desu yo, and another, more emphatically, iin desu yo, or with a bow, honto ni, ii no desu yo. Fun, no? However, I dislike the sono here not indicating "that test", but colloquially, two boys just leaving a test might just say "the test" since which test was known to them already, and Japanese is big in ambiguity being resolved by context rather than pointing, both literally and figuratively