Mark two MPI_Comm_idup related tests as xfail

They are known to be failing. Mark them as xfail so they will not send
false alarms to other patches.

No reviewer.

test/mpi/threads/comm/testlist

@@ -3,5 +3,5 @@ dup_leak_test 2
 comm_dup_deadlock 4
 comm_create_threads 4
 comm_create_group_threads 4
-comm_idup 4 mpiversion=3.0
-ctxidup 4
+comm_idup 4 mpiversion=3.0 xfail=ticket2108
+ctxidup 4 mpiversion=3.0 xfail=ticket2108