Synchronize capture queue before verifying mock expectations

RTCCameraVideoCapturerTestsWithMockedCaptureSession tests
startCaptureWithDevice and stopCapture, which dispatch asynchronously
to RTCDispatcherTypeCaptureSession. OCMVerifyAllWithDelay does not
reliably wait for these background tasks to complete. This change
explicitly synchronizes the capture dispatch queue before verification
to ensure deterministic test completion and prevent deallocation of
unverified strict mock objects.

Bug: none
Change-Id: I94c64456291e970bcef962d4a78ae378c4e8f658
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/472500
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Auto-Submit: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47713}

sdk/objc/unittests/RTCCameraVideoCapturerTests.mm

diff --git a/sdk/objc/unittests/RTCCameraVideoCapturerTests.mm b/sdk/objc/unittests/RTCCameraVideoCapturerTests.mm
index d5b4da9..2d91532 100644
--- a/sdk/objc/unittests/RTCCameraVideoCapturerTests.mm
+++ b/sdk/objc/unittests/RTCCameraVideoCapturerTests.mm
@@ -18,7 +18,7 @@
 #import "base/RTCVideoFrame.h"
 #import "components/capturer/RTCCameraVideoCapturer.h"
 #import "helpers/AVCaptureSession+DevicePosition.h"
-#import "helpers/RTCDispatcher.h"
+#import "helpers/RTCDispatcher+Private.h"
 #import "helpers/scoped_cftyperef.h"
 
 #define WAIT(timeoutMs)                                                        \
@@ -476,6 +476,10 @@
   [self.capturer stopCapture];
 
   // Start capture code is dispatched async.
+  dispatch_sync([RTC_OBJC_TYPE(RTCDispatcher)
+                    dispatchQueueForType:RTCDispatcherTypeCaptureSession],
+                ^{
+                });
   OCMVerifyAllWithDelay(_captureSessionMock, 15);
 }
 
@@ -489,6 +493,10 @@
   [self.capturer startCaptureWithDevice:self.deviceMock format:format fps:30];
 
   // Start capture code is dispatched async.
+  dispatch_sync([RTC_OBJC_TYPE(RTCDispatcher)
+                    dispatchQueueForType:RTCDispatcherTypeCaptureSession],
+                ^{
+                });
   OCMVerifyAllWithDelay(self.deviceMock, 15);
 }