[test] [support] Use socket()+bind() to create unix sockets portably

Replace the mknod() call with socket() + bind() for creating unix
sockets.  The mknod() method is not portable and does not work
on NetBSD while binding the socket should work on all systems supporting
unix sockets.

Differential Revision: https://reviews.llvm.org/D55576

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@349305 91177308-0d34-0410-b5e6-96231b3b80d8