xdot: fix tests
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.

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