cpansearch: secure test

- swap CPAN mirror to a secure one (the one used in formulae)
- swap short option to the long form, because the former ignores
  any URL argument passed

Formula/cpansearch.rb

@@ -23,7 +23,7 @@ class Cpansearch < Formula
   end
 
   test do
-    output = shell_output("#{bin}/cpans -f http://cpan.nctu.edu.tw/")
+    output = shell_output("#{bin}/cpans --fetch https://cpan.metacpan.org/")
     assert_match "packages recorded", output
   end
 end