Better Debug for Vars and VarsOs

Display actual vars instead of two dots.

The same was done for Args and ArgsOs in 275f9a04af.

library/std/src/env/tests.rs

@@ -95,8 +95,28 @@ fn args_debug() {
         format!("Args {{ inner: {:?} }}", args().collect::<Vec<_>>()),
         format!("{:?}", args())
     );
+}
+
+#[test]
+fn args_os_debug() {
     assert_eq!(
         format!("ArgsOs {{ inner: {:?} }}", args_os().collect::<Vec<_>>()),
         format!("{:?}", args_os())
     );
 }
+
+#[test]
+fn vars_debug() {
+    assert_eq!(
+        format!("Vars {{ inner: {:?} }}", vars().collect::<Vec<_>>()),
+        format!("{:?}", vars())
+    );
+}
+
+#[test]
+fn vars_os_debug() {
+    assert_eq!(
+        format!("VarsOs {{ inner: {:?} }}", vars_os().collect::<Vec<_>>()),
+        format!("{:?}", vars_os())
+    );
+}