feat: improve watch execution mode

The `watch` command now requires user action to move to the next
exercise.

BREAKING CHANGE: this changes the behavior of `watch`.

exercises/test3.rs

@@ -7,6 +7,8 @@
 // we expect to get when we call `times_two` with a negative number.
 // No hints, you can do this :)
 
+// I AM NOT DONE
+
 pub fn times_two(num: i32) -> i32 {
     num * 2
 }