Mark tests as slow to enable separate running

[effigies]

Running locally (Python 3.12), these tests take >10s:

143.71s call     nitransforms/tests/test_surface.py::test_ProjectUnproject
61.60s call     nitransforms/tests/test_surface.py::test_SurfaceResampler
12.49s call     nitransforms/tests/test_resampling.py::test_apply_transformchain

On GHA (Python 3.12):

  273.08s call     nitransforms/tests/test_surface.py::test_ProjectUnproject
  101.45s call     nitransforms/tests/test_surface.py::test_SurfaceResampler

It would be good to mark these as slow, exclude from the default runs, and set up a separate CI job. This is the majority of the test runtime.