matrix-synapse: expose nixos-test in derivation

As proposed during the last matrix-synapse bump[1].

[1] https://github.com/NixOS/nixpkgs/pull/75860#issuecomment-567007810

pkgs/servers/matrix-synapse/default.nix

@@ -1,5 +1,5 @@
 { lib, stdenv, python3, openssl
-, enableSystemd ? stdenv.isLinux
+, enableSystemd ? stdenv.isLinux, nixosTests
 }:
 
 with python3.pkgs;
@@ -78,6 +78,8 @@ in buildPythonApplication rec {
 
   doCheck = !stdenv.isDarwin;
 
+  passthru.tests = { inherit (nixosTests) matrix-synapse; };
+
   checkPhase = ''
     PYTHONPATH=".:$PYTHONPATH" ${python3.interpreter} -m twisted.trial tests
   '';