[Concurrency] disable async_taskgroup_is_asyncsequence on windows

It seems this test not only has issues on Linux but also windows.
https://ci-external.swift.org/job/oss-swift-windows-x86_64-vs2019/4488/console


Disable for now, until we're able to fix it.

Author: ktoso

test/Concurrency/Runtime/async_taskgroup_is_asyncsequence.swift

@@ -3,6 +3,7 @@
 // REQUIRES: executable_test
 // REQUIRES: concurrency
 // XFAIL: linux
+// XFAIL: windows
 
 #if canImport(Darwin)
 import Darwin