patch 8.2.4228: no tests for clicking in the GUI tabline

Problem:    No tests for clicking in the GUI tabline.
Solution:   Add test functions to generate the events.  Add tests using the
            functions. (Yegappan Lakshmanan, closes #9638)
diff --git a/src/testdir/test_normal.vim b/src/testdir/test_normal.vim
index 4a09964..9fcc9fd 100644
--- a/src/testdir/test_normal.vim
+++ b/src/testdir/test_normal.vim
@@ -891,6 +891,7 @@
 func Test_normal_z_error()
   call assert_beeps('normal! z2p')
   call assert_beeps('normal! zq')
+  call assert_beeps('normal! cz1')
 endfunc
 
 func Test_normal15_z_scroll_vert()
@@ -930,7 +931,7 @@
   call assert_equal(10, winheight(0))
   exe "norm! z12\<cr>"
   call assert_equal(12, winheight(0))
-  exe "norm! z10\<cr>"
+  exe "norm! z15\<Del>0\<cr>"
   call assert_equal(10, winheight(0))
 
   " Test for z.