Migrate test for spm_px_to_nm_scaling

[ns-rse]

The AFMReader.io.spm_px_to_nm_scaling() function had additional units migrated to AFMReader (see #84).

We don't have the test copied over from TopoStats (tests/test_io.py)...

@patch("pySPM.SPM.SPM_image.pxs")
@pytest.mark.parametrize(
    ("unit", "x", "y", "expected_px2nm"),
    [
        pytest.param("mm", 0.01, 0.01, 10000, id="mm units; square"),
        pytest.param("um", 1.5, 1.5, 1500, id="um units; square"),
        pytest.param("nm", 50, 50, 50, id="nm units; square"),
        pytest.param("pm", 233, 233, 0.233, id="pm units; square"),
        pytest.param("pm", 1, 512, 0.001, id="pm units; rectangular (thin)"),
        pytest.param("pm", 512, 1, 0.512, id="pm units; rectangular (tall)"),
    ],
)
def test__spm_pixel_to_nm_scaling(
    mock_pxs,
    load_scan_spm: LoadScans,
    spm_channel_data: pySPM.SPM.SPM_image,
    unit: str,
    x: int,
    y: int,
    expected_px2nm: float,
) > None:
    """Test extraction of pixels to nanometer scaling."""
    mock_pxs.return_value = [(x, unit), (y, unit)]  # issue is that pxs is a func that returns the data
    result = load_scan_spm._spm_pixel_to_nm_scaling(spm_channel_data)
    assert result == expected_px2nm