Try to shutdown Fluent "more" when closing the viewer during testing

Even with PR 18280 fixed, we still *occasionally* see l10n-related errors when closing the integration-tests on the bots.

web/app.js

@@ -2111,16 +2111,15 @@ const PDFViewerApplication = {
    * @ignore
    */
   async testingClose() {
-    this.l10n?.pause();
-    this.findBar?.close();
-
     this.unbindEvents();
     this.unbindWindowEvents();
 
     this._globalAbortController?.abort();
     this._globalAbortController = null;
 
-    await this.close();
+    this.findBar?.close();
+
+    await Promise.all([this.l10n?.destroy(), this.close()]);
   },
 
   _accumulateTicks(ticks, prop) {

web/l10n.js

@@ -23,6 +23,8 @@
 class L10n {
   #dir;
 
+  #elements = new Set();
+
   #lang;
 
   #l10n;
@@ -69,6 +71,7 @@ class L10n {
 
   /** @inheritdoc */
   async translate(element) {
+    this.#elements.add(element);
     try {
       this.#l10n.connectRoot(element);
       await this.#l10n.translateRoots();
@@ -77,6 +80,22 @@ class L10n {
     }
   }
 
+  /** @inheritdoc */
+  async destroy() {
+    for (const element of this.#elements) {
+      this.#l10n.disconnectRoot(element);
+    }
+    this.#elements.clear();
+    this.#l10n.pauseObserving();
+
+    // Since `disconnectRoot`/`pauseObserving` can still trigger asynchronous
+    // operations, without any way to actually cancel them, attempt to
+    // workaround timing issues when closing the integration-tests.
+    await new Promise(resolve => {
+      window.requestAnimationFrame(resolve);
+    });
+  }
+
   /** @inheritdoc */
   pause() {
     this.#l10n.pauseObserving();