Add more unit tests to atmos_phys_pbl_utils

During the review of #347, @nusbaume wrote:
> We should also note that this function needs unit tests in a new issue as well (which could be combined with the Brunt-Vaisala issue).

I think we could add unit tests to
* calc_brunt_vaisala_frequency_squared
* calc_bulk_richardson_number
* calc_vertical_shear_squared