patch 8.2.1799: some Normal mode commands not fully tested

Problem:    Some Normal mode commands not fully tested.
Solution:   Add a few more tests. (Yegappan Lakshmanan, closes #7073)
diff --git a/src/testdir/test_registers.vim b/src/testdir/test_registers.vim
index f71e418..521f8ad 100644
--- a/src/testdir/test_registers.vim
+++ b/src/testdir/test_registers.vim
@@ -274,6 +274,9 @@
   call assert_fails('let r = getreg("=", [])', 'E745:')
   call assert_fails('let r = getreg("=", 1, [])', 'E745:')
   enew!
+
+  " Using a register in operator-pending mode should fail
+  call assert_beeps('norm! c"')
 endfunc
 
 func Test_set_register()