[test] Mark taskgroup tests XFAIL like Linux.

The pr #35755 changed these tests from `REQUIRES: OS=macosx` to using
`XFAIL: linux`, which suggests other platforms supporting concurrency
must affirmatively mark these failing tests XFAIL as well.

This is distinct from #35759, since ideally that pr will be cherrypicked
over to the 5.4 branch and including these changes there would cause a
prospective cherrypick to diverge.

Author: 3405691582

test/Concurrency/Runtime/async_taskgroup_add_handle_completion.swift

@@ -3,6 +3,7 @@
 // REQUIRES: concurrency
 // XFAIL: windows
 // XFAIL: linux
+// XFAIL: openbsd
 
 import Dispatch
 import Darwin

test/Concurrency/Runtime/async_taskgroup_is_empty.swift

@@ -3,6 +3,7 @@
 // REQUIRES: concurrency
 // XFAIL: windows
 // XFAIL: linux
+// XFAIL: openbsd
 
 import Dispatch
 

test/Concurrency/Runtime/async_taskgroup_next_not_invoked_cancelAll.swift

@@ -3,6 +3,7 @@
 // REQUIRES: concurrency
 // XFAIL: windows
 // XFAIL: linux
+// XFAIL: openbsd
 
 import Dispatch
 

test/Concurrency/Runtime/async_taskgroup_next_not_invoked_without_cancelAll.swift

@@ -3,6 +3,7 @@
 // REQUIRES: concurrency
 // XFAIL: windows
 // XFAIL: linux
+// XFAIL: openbsd
 
 import func Foundation.sleep
 

test/Concurrency/Runtime/async_taskgroup_throw_recover.swift

@@ -3,6 +3,7 @@
 // REQUIRES: concurrency
 // XFAIL: windows
 // XFAIL: linux
+// XFAIL: openbsd
 
 import Dispatch