[libc++] Fix incorrect definition of TEST_HAS_C11_FEATURES

Summary:
The macro was not defined in C++11 mode when it should have been, at least
according to how _LIBCPP_HAS_C11_FEATURES is defined.

Reviewers: mclow.lists, EricWF, jfb, dexonsmith

Subscribers: christof, dexonsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D50719

llvm-svn: 339702