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/src/evalfunc.c b/src/evalfunc.c
index 7800aa9..67875fe 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -6629,6 +6629,10 @@
 	else if (STRICMP(name, "conpty") == 0)
 	    n = use_conpty();
 #endif
+#ifdef FEAT_CLIPBOARD
+	else if (STRICMP(name, "clipboard_working") == 0)
+	    n = clip_star.available;
+#endif
     }
 
     rettv->vval.v_number = n;