patch 8.1.2079: popup window test fails without +terminal

Problem:    Popup window test fails without +terminal.
Solution:   Check for the +terminal feature.
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index dfdfc13..b9d6d83 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -1993,6 +1993,8 @@
 endfunc
 
 func Test_popupwin_terminal_buffer()
+  CheckFeature terminal
+
   let ptybuf = term_start(&shell, #{hidden: 1})
   call assert_fails('let winnr = popup_create(ptybuf, #{})', 'E278:')
   exe 'bwipe! ' .. ptybuf