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`.

src/run.rs

@@ -5,7 +5,9 @@ use indicatif::ProgressBar;
 
 pub fn run(exercise: &Exercise) -> Result<(), ()> {
     match exercise.mode {
-        Mode::Test => test(exercise)?,
+        Mode::Test => {
+            test(exercise)?;
+        }
         Mode::Compile => compile_and_run(exercise)?,
     }
     Ok(())