mariadb-galera: add passthru.tests

pkgs/servers/sql/mariadb/galera/default.nix

···
 { lib, stdenv, fetchFromGitHub, buildEnv
 , asio, boost, check, openssl, cmake
+, nixosTests
 }:
 
 stdenv.mkDerivation rec {
···
     mkdir $out/lib/galera
     ln -s $out/lib/libgalera_smm.so $out/lib/galera/libgalera_smm.so
   '';
+
+  passthru.tests = {
+    inherit (nixosTests) mariadb-galera;
+  };
 
   meta = with lib; {
     description = "Galera 3 wsrep provider library";