[windows][vs2017] Mark flaky test as unsupported. (#36530)

The test has been flaky for the last couple of weeks. It almost always
fails, but from time to time the Windows VS2017 CI machine passes the
test.

Mark it as unsupported (since XFAIL will create random failures when the
test pass), to avoid noise in the CI machine.

Author: drodriguez

test/Concurrency/Runtime/async_task_handle_cancellation.swift

@@ -3,6 +3,9 @@
 // REQUIRES: executable_test
 // REQUIRES: concurrency
 
+// This test is flaky on VS2017 (unknown reasons)
+// UNSUPPORTED: MSVC_VER=15.0
+
 @main struct Main {
   static func main() async {
     let handle = Task.runDetached {