gnu-prolog: add test

Closes Homebrew/homebrew#35325.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Formula/gnu-prolog.rb

@@ -1,9 +1,7 @@
-require 'formula'
-
 class GnuProlog < Formula
-  homepage 'http://www.gprolog.org/'
-  url 'http://gprolog.univ-paris1.fr/gprolog-1.4.4.tar.gz'
-  sha1 '658b0efa5d916510dcddbbd980d90bc4d43a6e58'
+  homepage "http://www.gprolog.org/"
+  url "http://gprolog.univ-paris1.fr/gprolog-1.4.4.tar.gz"
+  sha1 "658b0efa5d916510dcddbbd980d90bc4d43a6e58"
 
   bottle do
     sha1 "8dc1b5782b6fb5e89c62fe3f5b07a908ddff6850" => :yosemite
@@ -19,11 +17,20 @@ class GnuProlog < Formula
   end
 
   def install
-    cd 'src' do
+    cd "src" do
       system "./configure", "--prefix=#{prefix}", "--with-doc-dir=#{doc}"
       ENV.deparallelize
       system "make"
       system "make", "install"
     end
   end
+
+  test do
+    (testpath/"test.pl").write <<-EOS.undent
+      :- initialization(main).
+      main :- write('Hello World!'), nl, halt.
+    EOS
+    system "#{bin}/gplc", "test.pl"
+    assert_match /Hello World!/, shell_output("./test")
+  end
 end