patch 8.2.3350: text properties test fails on MS-Windows

Problem:    Text properties test fails on MS-Windows.
Solution:   Set fileformat to unix.
diff --git a/src/testdir/test_textprop.vim b/src/testdir/test_textprop.vim
index 847968c..6af47e3 100644
--- a/src/testdir/test_textprop.vim
+++ b/src/testdir/test_textprop.vim
@@ -812,6 +812,7 @@
   bwipe!
 
   new
+  setlocal ff=unix
   call setline(1, range(500))
   call assert_equal(1491, line2byte(401))
   call prop_add(2, 1, {'type': 'comment'})