patch 8.1.1668: popup window test is a bit flaky on some systems

Problem:    Popup window test is a bit flaky on some systems.
Solution:   Clear the command line. (Naruhiko Nishino, closes #4656)

src/testdir/test_popupwin.vim

@@ -525,6 +525,9 @@ func Test_popup_select()
 
   call term_sendkeys(buf, ":call popup_close(winid)\<CR>")
   call term_sendkeys(buf, "\"*p")
+  " clean the command line, sometimes it still shows a command
+  call term_sendkeys(buf, ":\<esc>")
+
   call VerifyScreenDump(buf, 'Test_popupwin_select_02', {})
 
   " clean up

src/version.c

@@ -777,6 +777,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1668,
 /**/
     1667,
 /**/