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/src/proto/popupwin.pro b/src/proto/popupwin.pro
index 4fa3013..02cdce3 100644
--- a/src/proto/popupwin.pro
+++ b/src/proto/popupwin.pro
@@ -40,6 +40,7 @@
 void f_popup_move(typval_T *argvars, typval_T *rettv);
 void f_popup_setoptions(typval_T *argvars, typval_T *rettv);
 void f_popup_getpos(typval_T *argvars, typval_T *rettv);
+void f_popup_list(typval_T *argvars, typval_T *rettv);
 void f_popup_locate(typval_T *argvars, typval_T *rettv);
 void f_popup_getoptions(typval_T *argvars, typval_T *rettv);
 int error_if_term_popup_window(void);