At least parse the switch_user_group test

It won't run, but it can at least be analysed to see if it works!

src/switch.rs

@@ -87,7 +87,9 @@ impl Drop for SwitchUserGuard {
 /// Safely switch user and group for the current scope.
 /// Requires root access.
 ///
-/// ```ignore
+/// ```no_run
+/// use users::switch::switch_user_group;
+///
 /// {
 ///     let _guard = switch_user_group(1001, 1001);
 ///     // current and effective user and group ids are 1001