Ensure setCompatibilityMode continues to restart if previously started.

This was previous behavior, but the previous commit which prevents restarts based on _steppingInterval truthiness broke it because clearInterval does not modify the value of _steppingInterval

src/engine/runtime.js

@@ -1763,6 +1763,7 @@ class Runtime extends EventEmitter {
         this.compatibilityMode = compatibilityModeOn;
         if (this._steppingInterval) {
             clearInterval(this._steppingInterval);
+            this._steppingInterval = null;
             this.start();
         }
     }