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_cmdline.vim b/src/testdir/test_cmdline.vim
index 5d772ea..4dca8ea 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -392,6 +392,7 @@
   call delete('Xtags')
   set tags&
 
+  call assert_fails("call getcompletion('\\\\@!\\\\@=', 'buffer')", 'E871:')
   call assert_fails('call getcompletion("", "burp")', 'E475:')
   call assert_fails('call getcompletion("abc", [])', 'E475:')
 endfunc