Tests: add a workaround for Windows

The Windows test regularly fails if the stdout is not flushed on
completion.  This increases the stability of the tests on Windows
(though there is another async related issue lurking that causes a
sporadic failure).

Author: compnerd

Tests/Functional/Asynchronous/Use/main.swift

@@ -9,6 +9,10 @@
     import XCTest
 #endif
 
+#if os(Windows)
+    import ucrt
+#endif
+
 actor TestActor {
 
     enum Errors: String, Error {
@@ -56,7 +60,11 @@ class AsyncAwaitTests: XCTestCase {
 
     override func setUp() async throws {}
 
-    override func tearDown() async throws {}
+    override func tearDown() async throws {
+#if os(Windows)
+        fflush(stdout)
+#endif
+    }
 
     // CHECK: Test Case 'AsyncAwaitTests.test_explicitFailures_withinAsyncTests_areReported' started at \d+-\d+-\d+ \d+:\d+:\d+\.\d+
     // CHECK: .*[/\\]Asynchronous[/\\]Use[/\\]main.swift:[[@LINE+3]]: error: AsyncAwaitTests.test_explicitFailures_withinAsyncTests_areReported : XCTAssertTrue failed -