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

src/testdir/dumps/Test_popup_textprop_corn_4.dump

@@ -9,4 +9,4 @@
 |5|3| @21|╚+0#0000001#ffd7ff255|═@9|╝| +0#0000000#ffffff0@38
 |5|4| @72
 |5@1| @72
-|1| |l|i|n|e| |l|e|s@1|;| |b|e|f|o|r|e| |#|3| @1|1| |s|e|c|o|n|d| |a|g|o| @20|4|6|,|1| @9|4|9|%| 
+|:| @55|4|6|,|1| @9|4|9|%| 

src/testdir/test_popupwin_textprop.vim

@@ -96,6 +96,7 @@ func Test_textprop_popup_corners()
   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

src/version.c

@@ -761,6 +761,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1935,
 /**/
     1934,
 /**/