test: fix flaky timing and timeout guards (#969)

Clears the timeout guards once each race settles, measures the 15,000-vehicle transit ingestion with main-thread CPU time instead of wall time, and times the region-timeout case from the boundary lookup with a 2 s ceiling, below the 3 s default budget, so an ignored budget still fails.

Co-authored-by: Sameh Khamis <[email protected]>

src/sdr/controller.test.mjs

@@ -357,10 +357,11 @@ test('WebUSB picker remains available when no authorized receiver matches', asyn
 
 /** Resolve with the promise's value, or 'timed out' after `ms`. */
 function within(promise, ms = 1_000) {
-  return Promise.race([
-    promise,
-    new Promise((resolve) => setTimeout(() => resolve('timed out'), ms)),
-  ]);
+  let timer;
+  const timeout = new Promise((resolve) => {
+    timer = setTimeout(() => resolve('timed out'), ms);
+  });
+  return Promise.race([promise, timeout]).finally(() => clearTimeout(timer));
 }
 
 /**