Fix some random Darwin test failures due to timing dependency. (#33761)

Some Darwin tests failed randomly if a subscription took longer than 10s to
establish.  Add test-only conditionals around the relevant code so we suppress
that behavior in tests.

src/darwin/Framework/CHIP/MTRDevice.mm

@@ -383,6 +383,7 @@ - (BOOL)unitTestPretendThreadEnabled:(MTRDevice *)device;
 - (void)unitTestSubscriptionPoolDequeue:(MTRDevice *)device;
 - (void)unitTestSubscriptionPoolWorkComplete:(MTRDevice *)device;
 - (void)unitTestClusterDataPersisted:(MTRDevice *)device;
+- (BOOL)unitTestSuppressTimeBasedReachabilityChanges:(MTRDevice *)device;
 @end
 #endif
 
@@ -2026,15 +2027,24 @@ - (void)_setupSubscriptionWithReason:(NSString *)reason
 
     MTR_LOG("%@ setting up subscription with reason: %@", self, reason);
 
-    // Set up a timer to mark as not reachable if it takes too long to set up a subscription
-    MTRWeakReference<MTRDevice *> * weakSelf = [MTRWeakReference weakReferenceWithObject:self];
-    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, static_cast<int64_t>(kSecondsToWaitBeforeMarkingUnreachableAfterSettingUpSubscription) * static_cast<int64_t>(NSEC_PER_SEC)), self.queue, ^{
-        MTRDevice * strongSelf = weakSelf.strongObject;
-        if (strongSelf != nil) {
-            std::lock_guard lock(strongSelf->_lock);
-            [strongSelf _markDeviceAsUnreachableIfNeverSubscribed];
-        }
-    });
+    bool markUnreachableAfterWait = true;
+#ifdef DEBUG
+    if (delegate && [delegate respondsToSelector:@selector(unitTestSuppressTimeBasedReachabilityChanges:)]) {
+        markUnreachableAfterWait = ![delegate unitTestSuppressTimeBasedReachabilityChanges:self];
+    }
+#endif
+
+    if (markUnreachableAfterWait) {
+        // Set up a timer to mark as not reachable if it takes too long to set up a subscription
+        MTRWeakReference<MTRDevice *> * weakSelf = [MTRWeakReference weakReferenceWithObject:self];
+        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, static_cast<int64_t>(kSecondsToWaitBeforeMarkingUnreachableAfterSettingUpSubscription) * static_cast<int64_t>(NSEC_PER_SEC)), self.queue, ^{
+            MTRDevice * strongSelf = weakSelf.strongObject;
+            if (strongSelf != nil) {
+                std::lock_guard lock(strongSelf->_lock);
+                [strongSelf _markDeviceAsUnreachableIfNeverSubscribed];
+            }
+        });
+    }
 
     [_deviceController
         getSessionForNode:_nodeID.unsignedLongLongValue

src/darwin/Framework/CHIPTests/TestHelpers/MTRDeviceTestDelegate.m

@@ -103,6 +103,15 @@ - (void)unitTestClusterDataPersisted:(MTRDevice *)device
     }
 }
 
+- (BOOL)unitTestSuppressTimeBasedReachabilityChanges:(MTRDevice *)device
+{
+    // Allowing time-based reachability changes just makes the tests
+    // non-deterministic and can lead to random failures.  Suppress them
+    // unconditionally for now.  If we ever add tests that try to exercise that
+    // codepath, we can make this configurable.
+    return YES;
+}
+
 @end
 
 @implementation MTRDeviceTestDelegateWithSubscriptionSetupOverride