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.

runtime/doc/eval.txt

@@ -2627,6 +2627,8 @@ popup_findpreview()		Number	get window ID of preview popup window
 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})

runtime/doc/popup.txt

@@ -198,6 +198,7 @@ Other:
 	|popup_getoptions()|	get current options for a popup
 	|popup_getpos()|	get actual position and size of a popup
 	|popup_locate()|	find popup window at a screen position
+	|popup_list()|		get list of all popups
 
 
 DETAILS						*popup-function-details*
@@ -423,6 +424,10 @@ popup_hide({id})						*popup_hide()*
 		Can also be used as a |method|: >
 			GetPopup()->popup_hide()
 
+popup_list()						 *popup_list()*
+		Return a List with the |window-ID| of all existing popups.
+
+
 popup_locate({row}, {col})				 *popup_locate()*
 		Return the |window-ID| of the popup at screen position {row}
 		and {col}.  If there are multiple popups the one with the