patch 8.1.2050: popup window test fails in some configurations

Problem:    Popup window test fails in some configurations. (James McCoy)
Solution:   Clear the command line.
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index de8a23f..6f7cae6 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -1764,6 +1764,7 @@
 
   " remove the minwidth and maxheight
   call term_sendkeys(buf, ":call popup_setoptions(winid, #{maxheight: 0, minwidth: 0})\<CR>")
+  call term_sendkeys(buf, ":\<CR>")
   call VerifyScreenDump(buf, 'Test_popupwin_scroll_10', {})
 
   " clean up