DEV: Enable ember-qunit's TestIsolationValidation feature (#18666)

This ensures ember's runloop is settled before starting the next test.

app/assets/javascripts/discourse/tests/test-boot-ember-cli.js

@@ -61,6 +61,7 @@ document.addEventListener("discourse-booted", () => {
     loadTests: false,
     startTests: !disableAutoStart,
     setupEmberOnerrorValidation: !skipCore,
+    setupTestIsolationValidation: true,
   });
 });