[tests] Update to use lit's now-integrated XFAIL handling.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@188960 91177308-0d34-0410-b5e6-96231b3b80d8
XFAIL 3 tests on darwin 11-12.  The tests have recently been modified, are correct, and pass with an updated libc++.dylib

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@188964 91177308-0d34-0410-b5e6-96231b3b80d8