patch 8.2.0351: terminal in popup test is still a bit flaky

Problem:    Terminal in popup test is still a bit flaky.
Solution:   Clear and redraw before opening the popup.
diff --git a/src/testdir/test_terminal.vim b/src/testdir/test_terminal.vim
index 1a9e1e3..8cbd6a6 100644
--- a/src/testdir/test_terminal.vim
+++ b/src/testdir/test_terminal.vim
@@ -2357,6 +2357,7 @@
   call writefile(lines, 'XtermPopup')
   let buf = RunVimInTerminal('-S XtermPopup', #{rows: 15})
   call term_wait(buf, 100)
+  call term_sendkeys(buf, "\<C-L>")
   call term_sendkeys(buf, ":call OpenTerm(0)\<CR>")
   call term_wait(buf, 100)
   call term_sendkeys(buf, ":\<CR>")