Add basic NixOS tests for TiMidity++

nixos/tests/all-tests.nix

···
   tika = runTest ./tika.nix;
   timescaledb = handleTest ./timescaledb.nix {};
   timezone = handleTest ./timezone.nix {};
+  timidity = handleTest ./timidity {};
   tinc = handleTest ./tinc {};
   tinydns = handleTest ./tinydns.nix {};
   tinyproxy = handleTest ./tinyproxy.nix {};

nixos/tests/timidity/basic.nix

···
+import ../make-test-python.nix (
+  { pkgs, ... }:
+  {
+
+    name = "timidity";
+
+    nodes.machine =
+      { pkgs, ... }:
+      {
+        environment.systemPackages = [ pkgs.timidity ];
+      };
+
+    testScript = ''
+      start_all ()
+
+      ## TiMidity++ is around.
+      machine.succeed("command -v timidity")
+    '';
+  }
+)

nixos/tests/timidity/default.nix

···
+inputs: {
+  basic = import ./basic.nix inputs;
+  with-vorbis = import ./with-vorbis.nix inputs;
+}

nixos/tests/timidity/with-vorbis.nix

···
+import ../make-test-python.nix (
+  { pkgs, ... }:
+  {
+
+    name = "timidity-with-vorbis";
+
+    nodes.machine =
+      { pkgs, ... }:
+      {
+        environment.systemPackages = with pkgs; [
+          (timidity.override { enableVorbis = true; })
+          ffmpeg # # for `ffprobe`
+        ];
+      };
+
+    testScript = ''
+      import json
+
+      start_all()
+
+      ## TiMidity++ is around and it claims to support Ogg Vorbis.
+      machine.succeed("command -v timidity")
+      machine.succeed("timidity --help | grep 'Ogg Vorbis'")
+
+      ## TiMidity++ manages to process a MIDI input and produces an Ogg Vorbis
+      ## output file. NOTE: the `timidity` CLI succeeds even when the input file
+      ## does not exist; hence our test for the output file's existence.
+      machine.succeed("cp ${./tam-lin.midi} tam-lin.midi")
+      machine.succeed("timidity -Ov tam-lin.midi && test -e tam-lin.ogg")
+
+      ## The output file has the expected characteristics.
+      metadata_as_text = machine.succeed("ffprobe -show_format -print_format json -i tam-lin.ogg")
+      metadata = json.loads(metadata_as_text)
+      assert metadata['format']['format_name'] == 'ogg', \
+          f"expected 'format_name' to be 'ogg', got '{metadata['format']['format_name']}'"
+      assert 37 <= float(metadata['format']['duration']) <= 38, \
+          f"expected 'duration' to be between 37s and 38s, got {metadata['format']['duration']}s"
+    '';
+  }
+)