patch 8.2.0748: cannot get a list of all popups

Problem:    Cannot get a list of all popups.
Solution:   Add popup_list().  Use it in the test runner.
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 8086bd5..8259e99 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -2627,6 +2627,8 @@
 popup_getoptions({id})		Dict	get options of popup window {id}
 popup_getpos({id})		Dict	get position of popup window {id}
 popup_hide({id})		none	hide popup menu {id}
+popup_list()			List	get a list of window IDs of al popups
+popup_locate({row}, {col})	Number 	get window ID of popup at position
 popup_menu({what}, {options})	Number	create a popup window used as a menu
 popup_move({id}, {options})	none	set position of popup window {id}
 popup_notification({what}, {options})