test__groupby_x_along_channels needs to test for value

[leewujung]

The current test__groupby_x_along_channels does not test for output values of _groupby_x_along_channels. The output values of this function are not tested in any other test functions either.

The test function also has an unused input argument lat_lon, which needs investigation to see if that is actually needed, or if that is a missed coverage issue.