blockbook: add link to nixos test

pkgs/servers/blockbook/default.nix

···
 , snappy
 , zeromq
 , zlib
+, nixosTests
 }:
 
 buildGoModule rec {
···
     cp -r $src/static/templates/ $out/share/
     cp -r $src/static/css/ $out/share/
   '';
+
+  passthru.tests = {
+    smoke-test = nixosTests.blockbook-frontend;
+  };
 
   meta = with lib; {
     description = "Trezor address/account balance backend";