fix test command execution

seems like i forgot that you actually need to run executables created
with `rustc --test`! this also reworks the "test1" exercise a bit

closes #108

exercises/test1.rs

@@ -5,16 +5,17 @@
 
 // Mary is buying apples. One apple usually costs 2 dollars, but if you buy
 // more than 40 at once, each apple only costs 1! Write a function that calculates
-// the price of an order of apples given the order amount.
+// the price of an order of apples given the order amount. No hints this time!
 
-fn main() {
+// Put your function here!
+// fn ..... {
+
+// Don't modify this function!
+#[test]
+fn verify_test() {
     let price1 = calculateprice(55);
     let price2 = calculateprice(40);
 
-    // Don't modify this!
-    if price1 == 55 && price2 == 80 {
-        println!("Good job!");
-    } else {
-        panic!("Uh oh! Wrong price!");
-    }
+    assert_eq!(price1, 55);
+    assert_eq!(price2, 80);
 }