mystmd: add version tester

pkgs/by-name/my/mystmd/package.nix

@@ -1,4 +1,4 @@
-{ lib, buildNpmPackage, fetchFromGitHub }:
+{ lib, buildNpmPackage, fetchFromGitHub, mystmd, testers }:
 
 buildNpmPackage rec {
   pname = "mystmd";
@@ -23,6 +23,11 @@ buildNpmPackage rec {
     runHook postInstall
   '';
 
+  passthru.tests.version = testers.testVersion {
+    package = mystmd;
+    version = "v${version}";
+  };
+
   meta = with lib; {
     description = "Command line tools for working with MyST Markdown";
     homepage = "https://github.com/executablebooks/mystmd";