patch 8.2.1729: endless loop when ":normal" feeds popup window filter

Problem:    Endless loop when ":normal" feeds popup window filter.
Solution:   Add the ex_normal_busy_done flag.

src/testdir/dumps/Test_popupwin_normal_cmd.dump

@@ -0,0 +1,10 @@
+> +0&#ffffff0@74
+|~+0#4040ff13&| @73
+|~| @73
+|~| @73
+|~| @31| +0#0000000&@8| +0#4040ff13&@32
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+| +0#0000000&@56|0|,|0|-|1| @8|A|l@1| 

src/testdir/test_popupwin.vim

@@ -1539,6 +1539,23 @@ func Test_popup_filter()
   call popup_clear()
 endfunc
 
+" this tests that the "ex_normal_busy_done" flag works
+func Test_popup_filter_normal_cmd()
+  CheckScreendump
+
+  let lines =<< trim END
+      let g:winid = popup_create('some text', {'filter': 'invalidfilter'})
+      call timer_start(0, {-> win_execute(g:winid, 'norm! zz')})
+  END
+  call writefile(lines, 'XtestPopupNormal')
+  let buf = RunVimInTerminal('-S XtestPopupNormal', #{rows: 10})
+  call TermWait(buf, 100)
+  call VerifyScreenDump(buf, 'Test_popupwin_normal_cmd', {})
+
+  call StopVimInTerminal(buf)
+  call delete('XtestPopupNormal')
+endfunc
+
 func ShowDialog(key, result)
   let s:cb_res = 999
   let winid = popup_dialog('do you want to quit (Yes/no)?', #{