patch 9.0.0148: a "below" aligned text property gets 'showbreak' displayed

Problem:    A "below" aligned text property gets 'showbreak' displayed.
Solution:   Do not use 'showbreak' before or in virtual text. (issue #10851)
diff --git a/src/testdir/test_textprop.vim b/src/testdir/test_textprop.vim
index 311de94..9bb3713 100644
--- a/src/testdir/test_textprop.vim
+++ b/src/testdir/test_textprop.vim
@@ -2311,6 +2311,7 @@
   CheckRunVimInTerminal
 
   let lines =<< trim END
+      set showbreak=+++
       call setline(1, 'some text here and other text there')
       call prop_type_add('rightprop', #{highlight: 'ErrorMsg'})
       call prop_type_add('afterprop', #{highlight: 'Search'})