test: abort the superseding navigation instead of leaving it in flight

The supersede assertion drove its second navigation with a fetch that never
settled, which left a navigation in flight for the rest of the page's life
holding the router's token and its own frame state. Under the full browser
suite that leak reded an unrelated file, the #1310 back-restore residue
assertion, on Firefox, while both files passed in isolation and while the
branch's own file passed everywhere.

Rejecting with an AbortError settles the navigation down the path the router
already takes for a superseded one, so the assertion observes the same thing
with nothing left running. Full browser suite green twice at this commit,
against a baseline that was green before the file was added.

Author: vivek7405

packages/core/test/routing/browser/nav-swipe-ab-levers.test.js

@@ -165,13 +165,23 @@ suite('Client router: back-swipe A/B levers (#1428)', () => {
       window.scrollTo = function (...args) { scrolls.push('scroll'); return origScrollTo.apply(this, args); };
       await navigate(location.origin + '/swipe-ab-superseded');
 
-      // Start a second navigation inside the deferred scroll's frame gap. Its
-      // fetch never settles, so it bumps the nav token and then does nothing
-      // else, which isolates the guard from anything the newer navigation
-      // would itself have done to scroll.
-      window.fetch = () => new Promise(() => {});
-      navigate(location.origin + '/swipe-ab-superseder');
+      // Start a second navigation inside the deferred scroll's frame gap. It
+      // bumps the nav token and then does nothing else, which isolates the
+      // guard from anything the newer navigation would itself have done to
+      // scroll.
+      //
+      // Aborted rather than left pending. A fetch that never settles leaves a
+      // navigation in flight for the rest of the page's life, holding the
+      // router's token and its own frame state, and a test that never cleans
+      // that up is a leak looking for somewhere to surface. An AbortError is
+      // the shape the router already treats as a superseded navigation, so it
+      // settles down the path it would take in production.
+      let abortPending;
+      window.fetch = () => new Promise((_, reject) => { abortPending = reject; });
+      const superseder = navigate(location.origin + '/swipe-ab-superseder');
       await settleFrames();
+      if (abortPending) abortPending(new DOMException('aborted', 'AbortError'));
+      await superseder.catch(() => {});
 
       // Without the token guard the first navigation's scroll fires here, into
       // a document a newer navigation already owns.