patch 8.1.1935: test for text property popup window is flaky

Problem:    Test for text property popup window is flaky.
Solution:   Remove the undo message
diff --git a/src/testdir/test_popupwin_textprop.vim b/src/testdir/test_popupwin_textprop.vim
index 4913815..2a4eb2c 100644
--- a/src/testdir/test_popupwin_textprop.vim
+++ b/src/testdir/test_popupwin_textprop.vim
@@ -96,6 +96,7 @@
   call VerifyScreenDump(buf, 'Test_popup_textprop_corn_3', {})
 
   call term_sendkeys(buf, "u")
+  call term_sendkeys(buf, ":\<CR>")
   call VerifyScreenDump(buf, 'Test_popup_textprop_corn_4', {})
 
   " clean up