patch 9.0.0975: virtual text below empty line misplaced when 'number' set

Problem:    Virtual text below an empty line is misplaced when 'number' is
            set.
Solution:   Adjust the computations. (closes #11629)
diff --git a/src/testdir/test_textprop.vim b/src/testdir/test_textprop.vim
index a6ed0af..480c9fb 100644
--- a/src/testdir/test_textprop.vim
+++ b/src/testdir/test_textprop.vim
@@ -2744,6 +2744,9 @@
   let buf = RunVimInTerminal('-S XscriptPropBelowAfterEmpty', #{rows: 8, cols: 60})
   call VerifyScreenDump(buf, 'Test_prop_below_after_empty_1', {}) 
 
+  call term_sendkeys(buf, ":set number\<CR>")
+  call VerifyScreenDump(buf, 'Test_prop_below_after_empty_2', {}) 
+
   call StopVimInTerminal(buf)
 endfunc