patch 8.2.2592: code coverage could be improved

Problem:    Code coverage could be improved.
Solution:   Add a few more tests. (Dominique Pellé, closes #7957)
diff --git a/src/testdir/test_normal.vim b/src/testdir/test_normal.vim
index 8aef41d..75b52bc 100644
--- a/src/testdir/test_normal.vim
+++ b/src/testdir/test_normal.vim
@@ -3207,6 +3207,13 @@
   " delete to a readonly register
   call setline(1, ['abcd'])
   call assert_beeps('normal ":d2l')
+
+  " D and d with 'nomodifiable'
+  call setline(1, ['abcd'])
+  setlocal nomodifiable
+  call assert_fails('normal D', 'E21:')
+  call assert_fails('normal d$', 'E21:')
+
   close!
 endfunc