oldtests: set shellslash for ":cd" test

src/nvim/testdir/test_cd.vim

@@ -24,7 +24,10 @@ func Test_cd_no_arg()
     call assert_equal(path, getcwd())
   else
     " Test that cd without argument echoes cwd on non-Unix systems.
+    let shellslash = &shellslash
+    set shellslash
     call assert_match(getcwd(), execute('cd'))
+    let &shellslash = shellslash
   endif
 endfunc