patch 8.2.2007: test for insert mode in popup is not reliable

Problem:    Test for insert mode in popup is not reliable.
Solution:   Wait for the popup to disappear. (Ozaki Kiichi, closes #7321)
diff --git a/src/testdir/test_terminal.vim b/src/testdir/test_terminal.vim
index ee0e743..cef379a 100644
--- a/src/testdir/test_terminal.vim
+++ b/src/testdir/test_terminal.vim
@@ -1266,7 +1266,7 @@
   call assert_equal('n', mode())
 
   call feedkeys("\<C-D>", 'xt')
-  sleep 50m
+  call WaitFor({-> popup_list() == []})
   delfunc StartTermInPopup
   iunmap <F3>
 endfunc