[tests] XFAIL a few things that require libc (?) support missing on Darwin.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@174450 91177308-0d34-0410-b5e6-96231b3b80d8
[tests] Mark some string.conversions expected failures (with system libc++).

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@174451 91177308-0d34-0410-b5e6-96231b3b80d8
[tests] Mark another stream input expected failure (with system libc++).

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@174452 91177308-0d34-0410-b5e6-96231b3b80d8
[tests] Mark another stream input expected failure (with system libc++).

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@174453 91177308-0d34-0410-b5e6-96231b3b80d8
[tests] If no explicit target triple is given, try to infer it.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@174454 91177308-0d34-0410-b5e6-96231b3b80d8
[tests] XFAIL some locale tests that don't seem to work on any Darwin.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@174459 91177308-0d34-0410-b5e6-96231b3b80d8