vim-patch:9.1.0539: Not enough tests for what v9.1.0535 fixed

Problem:  Not enough tests for what v9.1.0535 fixed
Solution: Add another test for ex-mode

This comes from: https://groups.google.com/g/vim_dev/c/F5-tDqoafz8/m/GqKF-uQsLD0J

related: vim/vim#15120

https://github.com/vim/vim/commit/248efab9b53516febf771bad9a88d3ca46f05ed6

Co-authored-by: Christian Brabandt <cb@256bit.org>

test/old/testdir/test_ex_mode.vim

@@ -359,6 +359,14 @@ func Test_backslash_multiline()
 \
 .', "xt")
   call assert_equal(["", "enum"], getline(1, 2))
+endfunc
+
+" Test using backslash in ex-mode after patch 9.1.0535
+func Test_backslash_multiline2()
+  throw 'Skipped: Nvim only supports Vim Ex mode'
+  new
+  call feedkeys('Qa
+X \\
 Y
 .', "xt")
   call assert_equal(['X \\', "Y"], getline(1, 2))