The child_added handler is called for each child, and there is no way inside it to know whether this was the last child. For that you'll want to listen to the value event.

So one options is:

let dbRef = firebase.database().ref("/ExamplePath");
let found = false;
dbRef.on("child_added", (child) => {
     if (child.val().test == testingVal) {
          console.log("Found One!");
          found = true;
     }
});

dbRef.on("value", (snapshot) => {
    if (found) {
         console.log("Finished!");
    }else{
         console.log("Nope!");
    }
});

This works great, and Firebase ensures the data gets only downloaded once, despite it being passed to both listeners.

If you prefer you can also do the same with just a value listener:

let dbRef = firebase.database().ref("/ExamplePath");

dbRef.on("value", (snapshot) => {
    let found = false;
    snapshot.forEach((child) => {
         if (child.val().test == testingVal) {
              console.log("Found One!");
              found = true;
         }
    });

    if (found) {
         console.log("Finished!");
    }else{
         console.log("Nope!");
    }
});

The result and efficiency of both approaches is the same, so you should choose whatever is most convenient for your app. For example, I prefer listening to child_ events when I'm responsible for updating each element in the UI directly, as the child_* event tells me pretty directly what to do.