feat(primitive_types6): Add a test (rust-lang#548)

Co-authored-by: Annika <[email protected]>
Co-authored-by: fmoko <[email protected]>

exercises/primitive_types/primitive_types6.rs

@@ -1,11 +1,16 @@
 // primitive_types6.rs
 // Use a tuple index to access the second element of `numbers`.
-// You can put this right into the `println!` where the ??? is.
+// You can put the expression for the second element where ??? is so that the test passes.
 // Execute `rustlings hint primitive_types6` for hints!
 
 // I AM NOT DONE
 
-fn main() {
+#[test]
+fn indexing_tuple() {
     let numbers = (1, 2, 3);
-    println!("The second number is {}", ???);
+    /// Replace below ??? with the tuple indexing syntax.
+    let second = ???;
+
+    assert_eq!(2, second
+        "This is not the 2nd number in the tuple!")
 }