test: Fix _top/_parent navigation crashing WTR in CI

WTR runs tests inside an iframe. Clicking links with target="_top" or
target="_parent" targets the WTR runner frame (canIntercept=false), so
calling e.intercept() unconditionally throws and lets the navigation
escape — crashing the entire test session.

Fix by:
- Checking canIntercept before calling e.intercept() in test handlers
- Skipping _top/_parent tests (like _blank) since they can't be
  meaningfully tested in an iframe context

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: sbesh91

test/router.test.js

@@ -575,10 +575,10 @@ describe('Router', () => {
 
 		let triedToNavigate = false;
 		const handler = (e) => {
-			e.intercept();
 			if (e['preact-iso-ignored']) {
 				triedToNavigate = true;
 			}
+			if (e.canIntercept) e.intercept();
 		}
 
 		const App = () => {
@@ -615,15 +615,11 @@ describe('Router', () => {
 			});
 		}
 
-		// TODO: These tests are rather flakey, for some reason the additional handler
-		// occasionally isn't running prior browser actually navigates away.
 		for (const target of shouldNavigate) {
 			it(`should allow default browser navigation for links with ${getName(target)}`, async () => {
-				// Currently cross-window navigations, (e.g., `target="_blank"`), do not trigger a
-				// `navigate` event, which makes this difficult to observe. Per the spec, however, this
-				// might be a bug in Chrome's implementation:
-				// https://github.com/WICG/navigation-api?tab=readme-ov-file#restrictions-on-firing-canceling-and-responding
-				if (target === '_blank' || target === '_BLANK' || target === 'custom') return;
+				// WTR runs tests in an iframe, so _top/_parent target the WTR runner frame
+				// (canIntercept=false). _blank/custom don't fire navigate events at all.
+				if (target === '_top' || target === '_parent' || target === '_blank' || target === '_BLANK' || target === 'custom') return;
 
 				scratch.querySelector(`#${createId(target)}`).click();
 				await sleep(1);
@@ -649,10 +645,10 @@ describe('Router', () => {
 
 		let triedToNavigate = false;
 		const handler = (e) => {
-			e.intercept();
 			if (e['preact-iso-ignored']) {
 				triedToNavigate = true;
 			}
+			if (e.canIntercept) e.intercept();
 		}
 
 		it('should support the `scope` prop (string)', async () => {