cowsay: add test

Closes Homebrew/homebrew#26593.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Formula/cowsay.rb

@@ -12,4 +12,11 @@ class Cowsay < Formula
     system "/bin/sh", "install.sh", prefix
     mv prefix/'man', share
   end
+
+  test do
+    output = `#{bin}/cowsay moo`
+    assert output.include?("moo")  # bubble
+    assert output.include?("^__^") # cow
+    assert_equal 0, $?.exitstatus
+  end
 end