add tests; refactor exercise links

exercises/test4.rs

@@ -0,0 +1,12 @@
+// test4.rs
+// This test covers the sections:
+// - Modules
+// - Macros
+
+// Write a macro that passes the test! No hints this time, you can do it!
+
+fn main() {
+    if my_macro!("world!") != "Hello world!" {
+        panic!("Oh no! Wrong output!");
+    }
+}