patch 8.2.3373: text property test fails on MS-Windows

Problem:    text property 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 f21fd40..4b58784 100644
--- a/src/testdir/test_textprop.vim
+++ b/src/testdir/test_textprop.vim
@@ -858,6 +858,7 @@
   bwipe!
 
   new
+  setlocal ff=unix
   call setline(1, range(520))
   call assert_equal(1491, line2byte(401))
   call prop_add(2, 1, {'type': 'comment'})