Fix flaky TestResourceManagerIsSafeForConcurrentAccessAndEnumeration timeout

The test uses Task.WaitAll with a 30-second timeout to wait for 10
threads concurrently enumerating resources. On loaded CI agents, 30s
is not always enough, causing Assert.True(false) with no useful
diagnostic.

Switch to await Task.WhenAll().WaitAsync(120s):
- WhenAll propagates actual thread-safety exceptions instead of
  swallowing them behind Assert.True(false)
- 120s gives 4x more headroom for slow CI agents while still
  providing a timeout safety net

This test has been filed as a Known Build Error three times (#80277,
#86013, #125448) across 3+ years.

Fixes #125448

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: lewing

src/libraries/System.Resources.Extensions/tests/BinaryResourceWriterUnitTest.cs

@@ -517,7 +517,7 @@ public static void EmbeddedResourcesAreUpToDate()
         [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsBinaryFormatterSupported))]
         [InlineData(false)]
         [InlineData(true)]
-        public static void TestResourceManagerIsSafeForConcurrentAccessAndEnumeration(bool useEnumeratorEntry)
+        public static async Task TestResourceManagerIsSafeForConcurrentAccessAndEnumeration(bool useEnumeratorEntry)
         {
             ResourceManager manager = new(
                 typeof(TestData).FullName,
@@ -534,7 +534,10 @@ public static void TestResourceManagerIsSafeForConcurrentAccessAndEnumeration(bo
                     TaskScheduler.Default))
                 .ToArray();
 
-            Assert.True(Task.WaitAll(tasks, TimeSpan.FromSeconds(30)));
+            Task allTasks = Task.WhenAll(tasks);
+            Task completed = await Task.WhenAny(allTasks, Task.Delay(TimeSpan.FromSeconds(120)));
+            Assert.True(completed == allTasks, "Timed out waiting for concurrent enumeration tasks");
+            await allTasks; // propagates any real exceptions
 
             void WaitForBarrierThenEnumerateResources()
             {