Ensure the mock URLSession waits until the main actor yields

Author: AlexNachbaur

Tests/TestCommon/URLSessionMock.swift

@@ -124,6 +124,7 @@ class URLSessionMock: URLSessionProtocol, @unchecked Sendable {
 
         return try await withCheckedThrowingContinuation { continuation in
             Task {
+                await MainActor.yield()
                 if let delay = requestDelay {
                     try await Task.sleep(delay: delay)
                 }