Add test for main dispatch queue processing in CFRunLoop

This ensures CFRunLoop correctly serves main dispatch queue.

Due to event specifics on Windows there was possibility to break
RunLoop making it think that main queue always has pending
work. The issue is adressed in swiftlang/swift-corelibs-libdispatch#542.

Author: lxbndr

Tests/Foundation/Tests/TestRunLoop.swift

@@ -125,6 +125,41 @@ class TestRunLoop : XCTestCase {
         XCTAssertTrue(didDeallocate)
     }
 
+    func test_mainDispatchQueueCallout() {
+        let runLoop = RunLoop.current
+
+        var asyncExecuted = false
+        DispatchQueue.main.async {
+            asyncExecuted = true
+        }
+
+        // RunLoop should service main queue
+        _ = runLoop.run(mode: .default, before: Date(timeIntervalSinceNow: 2))
+        XCTAssertTrue(asyncExecuted, "Main queue async code should be executed")
+
+        asyncExecuted = false
+        DispatchQueue.main.async {
+            asyncExecuted = true
+        }
+
+        // Second run to be sure RunLoop will not stuck
+        _ = runLoop.run(mode: .default, before: Date(timeIntervalSinceNow: 2))
+        XCTAssertTrue(asyncExecuted, "Main queue async code should be executed")
+
+        var timerFired = false
+        let dummyTimer = Timer.scheduledTimer(withTimeInterval: 0.5, repeats: false) { _ in
+            timerFired = true
+        }
+        runLoop.add(dummyTimer, forMode: .default)
+
+        // At this moment RunLoop has no work to do except waiting for timer.
+        // But RunLoop will exit prematurely if event from previous async calls
+        // got stuck in wrong state.
+        _ = runLoop.run(mode: .default, before: Date(timeIntervalSinceNow: 2))
+        
+        XCTAssertTrue(timerFired, "Time should fire already")
+    }
+
     static var allTests : [(String, (TestRunLoop) -> () throws -> Void)] {
         return [
             ("test_constants", test_constants),
@@ -134,6 +169,7 @@ class TestRunLoop : XCTestCase {
             ("test_runLoopLimitDate", test_runLoopLimitDate),
             ("test_runLoopPoll", test_runLoopPoll),
             ("test_addingRemovingPorts", test_addingRemovingPorts),
+            ("test_mainDispatchQueueCallout", test_mainDispatchQueueCallout)
         ]
     }
 }