Refactor the tests pull out common parts to pytest fixtures

[samaloney]

A lot of the test are repetitive leaving more room for errors in the test themselves for example the test_dft_idft_map, test_dft_idft_map_pixel_size, test_dft_idft_map_center and test_dft_idft_map_shape_pixel_size could all be combined into one test.

@pytest.mark.parametrize('shape', [(3, 3), (2, 3), (3, 2), (2, 2)])
@pytest.mark.parametrize('pixel_size', [(1, 1), (1, 2), (2, 1), (2, 2)])
@pytest.mark.parametrize('center', [(0, 0), (-1, 1), (1, -1), (1, 1)])
def test_dft_idft(shape, pixel_size, center):
    data = np.arange(np.prod(shape)).reshape(shape)
    uu = generate_uv(shape[0] * apu.pix, phase_centre=0 * apu.arcsec, pixel_size=pixel_size[0] * apu.arcsec / apu.pix)
    vv = generate_uv(shape[1] * apu.pix, phase_centre=0 * apu.arcsec, pixel_size=pixel_size[1] * apu.arcsec / apu.pix)
    u, v = np.meshgrid(uu, vv, indexing='ij')
    u = u.flatten()
    v = v.flatten()

    vis = dft_map(data, u=u, v=v, pixel_size=pixel_size * apu.arcsec / apu.pix)
    img = idft_map(vis, u=u, v=v, shape=shape * apu.pix, pixel_size=pixel_size * apu.arcsec / apu.pix)
    img = np.around(img / np.prod(shape))
    assert_allclose(data, img)