patch 8.2.5084: when the GUI shows a dialog tests get stuck

Problem:    When the GUI shows a dialog tests get stuck.
Solution:   Add the --gui-dialog-file argument.

src/testdir/runtest.vim

@@ -245,6 +245,12 @@ func RunTheTest(test)
     call popup_clear(1)
   endif
 
+  if filereadable('guidialogfile')
+    call add(v:errors, "Unexpected dialog:")
+    call add(v:errors, readfile('guidialogfile').join('\n'))
+    call delete('guidialogfile')
+  endif
+
   " Close any extra tab pages and windows and make the current one not modified.
   while tabpagenr('$') > 1
     let winid = win_getid()