xdot: fix tests

`nix-shell -p xdot` was failing, with an error like:

    builder for '/nix/store/n3yjqssn531c52sbkmpqvczmy5c0lm8n-python3.8-xdot-1.2.drv' failed with exit code 1; last 10 log lines:
          import numpy
      ModuleNotFoundError: No module named 'numpy'

Adding numpy uncovered another error, about not being able to initialize
gtk:

    error: Test failed: <unittest.runner.TextTestResult run=1 errors=1 failures=0>
    error: --- Error -------------------------------------------------------------------------------------------------------------------------------------------- nix-shell
    builder for '/nix/store/brz6q1ri12z51z3l2p5d6ny2jsf3qkjg-python3.8-xdot-1.2.drv' failed with exit code 1; last 10 log lines:
          raise RuntimeError(
      RuntimeError: Gtk couldn't be initialized. Use Gtk.init_check() if you want to handle this case.

pkgs/development/python-modules/xdot/default.nix

@@ -1,5 +1,5 @@
-{ lib, buildPythonPackage, fetchPypi, isPy3k
-, wrapGAppsHook, gobject-introspection, pygobject3, graphviz, gtk3 }:
+{ lib, buildPythonPackage, fetchPypi, isPy3k, python3, xvfb_run, stdenv
+, wrapGAppsHook, gobject-introspection, pygobject3, graphviz, gtk3, numpy }:
 
 buildPythonPackage rec {
   pname = "xdot";
@@ -11,9 +11,16 @@ buildPythonPackage rec {
   };
 
   disabled = !isPy3k;
-
   nativeBuildInputs = [ wrapGAppsHook ];
-  propagatedBuildInputs = [ gobject-introspection pygobject3 graphviz gtk3 ];
+  propagatedBuildInputs = [ gobject-introspection pygobject3 graphviz gtk3 numpy ];
+  checkInputs = [ xvfb_run ];
+
+  checkPhase = ''
+    xvfb-run -s '-screen 0 800x600x24' ${python3.interpreter} nix_run_setup test
+  '';
+
+  # https://github.com/NixOS/nixpkgs/pull/107872#issuecomment-752175866
+  doCheck = stdenv.isLinux;
 
   meta = with lib; {
     description = "An interactive viewer for graphs written in Graphviz's dot";