patch 8.1.1837: popup test fails if clipboard is supported but not working

Problem:    Popup test fails if clipboard is supported but not working.
Solution:   Add the "clipboard_working" feature. Also use Check commands
            instead of "if" and "throw".  And remove stray ch_logfile().
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 62bcfc2..4bc4287 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -9603,6 +9603,7 @@
 cindent			Compiled with 'cindent' support.
 clientserver		Compiled with remote invocation support |clientserver|.
 clipboard		Compiled with 'clipboard' support.
+clipboard_working	Compiled with 'clipboard' support and it can be used.
 cmdline_compl		Compiled with |cmdline-completion| support.
 cmdline_hist		Compiled with |cmdline-history| support.
 cmdline_info		Compiled with 'showcmd' and 'ruler' support.