gnu-barcode: add test

Closes Homebrew/homebrew#35383.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Formula/gnu-barcode.rb

@@ -1,10 +1,8 @@
-require 'formula'
-
 class GnuBarcode < Formula
-  homepage 'http://www.gnu.org/software/barcode/'
+  homepage "http://www.gnu.org/software/barcode/"
-  url 'http://ftpmirror.gnu.org/barcode/barcode-0.98.tar.gz'
+  url "http://ftpmirror.gnu.org/barcode/barcode-0.98.tar.gz"
-  mirror 'http://ftp.gnu.org/gnu/barcode/barcode-0.98.tar.gz'
+  mirror "http://ftp.gnu.org/gnu/barcode/barcode-0.98.tar.gz"
-  sha1 '15b9598bcaa67bcff1f63309d1a18840b9a12899'
+  sha1 "15b9598bcaa67bcff1f63309d1a18840b9a12899"
 
   def install
     system "./configure", "--disable-debug",
@@ -16,4 +14,10 @@ class GnuBarcode < Formula
                    "INFODIR=#{info}",
                    "install"
   end
+
+  test do
+    (testpath/"test.txt").write("12345")
+    system "#{bin}/barcode", "-e", "CODE39", "-i", "test.txt", "-o", "test.ps"
+    assert File.read("test.ps").start_with?("")
+  end
 end