patch 8.0.1510: cannot test if a command causes a beep

Problem:    Cannot test if a command causes a beep.
Solution:   Add assert_beeps().
diff --git a/src/testdir/test_normal.vim b/src/testdir/test_normal.vim
index 27c5897..2d1446b 100644
--- a/src/testdir/test_normal.vim
+++ b/src/testdir/test_normal.vim
@@ -2177,6 +2177,8 @@
 
 func! Test_normal45_drop()
   if !has('dnd')
+    " The ~ register does not exist
+    call assert_beeps('norm! "~')
     return
   endif