Filesystems are not required to maintain a hard link count consistent
with number of subdirectories. For example, on btrfs all directories
have nlink==1. Account for that in the test.
Differential Revision: https://reviews.llvm.org/D29706
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@294431 91177308-0d34-0410-b5e6-96231b3b80d8