patch 8.2.1046: insufficient tests for src/buffer.c

Problem:    Insufficient tests for src/buffer.c.
Solution:   Add more tests.  Move comments related tests to a separate file.
            (Yegappan Lakshmanan, closes #6325)
diff --git a/src/testdir/test_normal.vim b/src/testdir/test_normal.vim
index 2a06bb5..a4dc619 100644
--- a/src/testdir/test_normal.vim
+++ b/src/testdir/test_normal.vim
@@ -1267,6 +1267,7 @@
   edit Xfoo | %bw
   call assert_fails(':buffer #', 'E86')
   call assert_fails(':execute "normal! \<C-^>"', 'E23')
+  call assert_fails("normal i\<C-R>#", 'E23:')
 
   " Test for the expected behavior when switching between two named buffers.
   edit Xfoo | edit Xbar