patch 8.1.1856: popup preview test fails sometimes

Problem:    popup preview test fails sometimes. (Christian Brabandt)
Solution:   Clear the command line.
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index 26ab870..2c055f0 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -2173,6 +2173,7 @@
   call term_sendkeys(buf, ":cd testdir\<CR>")
 
   call term_sendkeys(buf, ":pclose\<CR>")
+  call term_sendkeys(buf, ":\<BS>")
   call VerifyScreenDump(buf, 'Test_popupwin_previewpopup_6', {})
 
   call term_sendkeys(buf, ":pedit +/theword Xtagfile\<CR>")