[test]Make test work on Windows

Previously, the test didn't work because '\' characters appeared in the
sed string, causing bogus escape characters to form in the substituted
string literal. Switching to using '%/p' causes the path to be emitted
with '/' characters instead, so that there are are no escaping issues.

Reviewed by: kzhuravl, grimar

Differential Revision: https://reviews.llvm.org/D61856

llvm-svn: 360660