mpd: add test & install example conf file

Follow up to Homebrew/legacy-homebrew#39353. The test should ensure we catch the need to
bump in future.

Closes Homebrew/legacy-homebrew#39360.

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

Formula/mpd.rb

@@ -4,7 +4,7 @@ class Mpd < Formula
 
   stable do
     url "http://www.musicpd.org/download/mpd/0.19/mpd-0.19.9.tar.xz"
-    sha1 "6683bee5f132eda318c5a61ec14b2df8d9164d60"
+    sha256 "47851423053cd38cfad65be5985b41b7cd5bdbe8d0d13378e11748a28b19f26f"
   end
 
   bottle do
@@ -70,6 +70,7 @@ def install
       --disable-debug
       --disable-dependency-tracking
       --prefix=#{prefix}
+      --sysconfdir=#{etc}
       --enable-bzip2
       --enable-ffmpeg
       --enable-fluidsynth
@@ -89,7 +90,9 @@ def install
     system "./configure", *args
     system "make"
     ENV.j1 # Directories are created in parallel, so let's not do that
-    system "make install"
+    system "make", "install"
+
+    (etc+"mpd").install "doc/mpdconf.example" => "mpd.conf"
   end
 
   plist_options :manual => "mpd"
@@ -116,4 +119,18 @@ def plist; <<-EOS.undent
     </plist>
     EOS
   end
+
+  test do
+    pid = fork do
+      exec "#{bin}/mpd --stdout --no-daemon --no-config"
+    end
+    sleep 2
+
+    begin
+      assert_match /OK MPD/, shell_output("curl localhost:6600")
+    ensure
+      Process.kill("SIGINT", pid)
+      Process.wait(pid)
+    end
+  end
 end