gnu-indent: add test

Closes Homebrew/homebrew#35310.

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

Formula/gnu-indent.rb

@@ -1,5 +1,3 @@
-require "formula"
-
 class GnuIndent < Formula
   homepage "https://www.gnu.org/software/indent/"
   url "http://ftpmirror.gnu.org/indent/indent-2.2.10.tar.gz"
@@ -28,6 +26,18 @@ class GnuIndent < Formula
     system "./configure", *args
     system "make", "install"
   end
+
+  test do
+    (testpath/"test.c").write("int main(){ return 0; }")
+    system "#{bin}/gindent", "test.c"
+    assert_equal File.read("test.c"), <<-EOS.undent
+      int
+      main ()
+      {
+        return 0;
+      }
+    EOS
+  end
 end
 
 __END__