[test] Fix reproduce-blackslash.s test with NetBSD tar

Unlike GNU tar and libarchive bsdtar, NetBSD 'tar -t' output does not
use C-style escapes and instead outputs paths literally.  Fix the test
to account both for escaped and literal backslash output.

Differential Revision: https://reviews.llvm.org/D55441

llvm-svn: 348628