Disable trivial pair copy/move tests when unsupported

Summary:
On FreeBSD, for ABI compatibility reasons, the pair trivial copy
constructor is disabled, using the aptly-named
`_LIBCPP_DEPRECATED_ABI_DISABLE_PAIR_TRIVIAL_COPY_CTOR` define.

Disable the related tests when this define is on, so they don't fail
unexpectedly.

Reviewers: emaste, rsmith, theraven, EricWF

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D25449

llvm-svn: 284047