patch 8.1.1856: popup preview test fails sometimes

Problem:    popup preview test fails sometimes. (Christian Brabandt)
Solution:   Clear the command line.

src/testdir/dumps/Test_popupwin_previewpopup_6.dump

@@ -11,4 +11,4 @@
 |v|e|r|y| |l|o|n|g| |l|i|n|e| |w|h|e|r|e| |t|h|e| |w|o|r|d| |i|s| |a|l|s|o| >a|n|o|t|h|e|r| @29
 |~+0#4040ff13&| @73
 |~| @73
-|:+0#0000000&|p|c|l|o|s|e| @49|1@1|,|3|9| @8|A|l@1| 
+| +0#0000000&@56|1@1|,|3|9| @8|A|l@1| 

src/testdir/test_popupwin.vim

@@ -2173,6 +2173,7 @@ func Test_previewpopup()
   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>")

src/version.c

@@ -769,6 +769,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1856,
 /**/
     1855,
 /**/