patch 8.2.0738: mouse handling in a terminal window not well tested

Problem:    Mouse handling in a terminal window not well tested.
Solution:   Add tests. (Yegappan Lakshmanan, closes #6052)

src/testdir/term_util.vim

@@ -127,6 +127,9 @@ func StopVimInTerminal(buf)
   " In Command-line it's inserted, the CTRL-U removes it again.
   call term_sendkeys(a:buf, "\<C-O>:\<C-U>qa!\<cr>")
 
+  " Wait for all the pending updates to terminal to complete
+  call TermWait(a:buf)
+
   call WaitForAssert({-> assert_equal("finished", term_getstatus(a:buf))})
   only!
 endfunc