patch 9.1.0500: cannot switch buffer in a popup

Problem:  cannot switch buffer in a popup
          (Yggdroot)
Solution: add popup_setbuf() function

fixes: #15006
closes: #15026

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_vim9_builtin.vim b/src/testdir/test_vim9_builtin.vim
index 6b1a50d..9adb7a5 100644
--- a/src/testdir/test_vim9_builtin.vim
+++ b/src/testdir/test_vim9_builtin.vim
@@ -3152,6 +3152,11 @@
   v9.CheckSourceDefAndScriptFailure(['popup_settext(1, 2)'], ['E1013: Argument 2: type mismatch, expected string but got number', 'E1222: String or List required for argument 2'])
 enddef
 
+def Test_popup_setbuf()
+  v9.CheckSourceDefAndScriptFailure(['popup_setbuf([], "abc")'], ['E1013: Argument 1: type mismatch, expected number but got list<any>', 'E1210: Number required for argument 1'])
+  v9.CheckSourceDefAndScriptFailure(['popup_setbuf(1, [])'], ['E1013: Argument 2: type mismatch, expected string but got list<any>', 'E1220: String or Number required for argument 2'])
+enddef
+
 def Test_popup_show()
   v9.CheckSourceDefAndScriptFailure(['popup_show("a")'], ['E1013: Argument 1: type mismatch, expected number but got string', 'E1210: Number required for argument 1'])
   v9.CheckSourceDefAndScriptFailure(['popup_show(true)'], ['E1013: Argument 1: type mismatch, expected number but got bool', 'E1210: Number required for argument 1'])