patch 8.1.1687: the evalfunc.c file is too big

Problem:    The evalfunc.c file is too big.
Solution:   Move testing support to a separate file.
diff --git a/src/proto.h b/src/proto.h
index 9119a52..269d81c 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -202,6 +202,7 @@
 #  include "popupwin.pro"
 #  include "textprop.pro"
 # endif
+# include "testing.pro"
 # include "ui.pro"
 # include "undo.pro"
 # include "usercmd.pro"