patch 8.2.0259: terminal in popup test sometimes fails

Problem:    Terminal in popup test sometimes fails.
Solution:   Clear the command line.
diff --git a/src/testdir/test_terminal.vim b/src/testdir/test_terminal.vim
index 993498c..5259454 100644
--- a/src/testdir/test_terminal.vim
+++ b/src/testdir/test_terminal.vim
@@ -2353,6 +2353,7 @@
 	\ ]
   call writefile(lines, 'XtermPopup')
   let buf = RunVimInTerminal('-S XtermPopup', #{rows: 15})
+  call term_sendkeys(buf, ":\<CR>")
   call VerifyScreenDump(buf, 'Test_terminal_popup_1', {})
 
   call term_sendkeys(buf, ":q\<CR>")