mpack: add passthru.tests.version

pkgs/by-name/mp/mpack/package.nix

@@ -2,6 +2,8 @@
   lib,
   stdenv,
   fetchurl,
+  testers,
+  mpack,
 }:
 
 stdenv.mkDerivation rec {
@@ -36,6 +38,16 @@ stdenv.mkDerivation rec {
 
   enableParallelBuilding = true;
 
+  passthru.tests = {
+    version = testers.testVersion {
+      command = ''
+        mpack 2>&1 || echo "mpack exited with error code $?"
+      '';
+      package = mpack;
+      version = "mpack version ${version}";
+    };
+  };
+
   meta = with lib; {
     description = "Utilities for encoding and decoding binary files in MIME";
     license = licenses.free;