DOC: Add testing dependencies to build workflow instructions (#21167)
DOC: Add testing dependencies to build workflow instructions (#21167)

Adds note on how to install the test dependencies when building numpy
from source.

Co-authored-by: Ross Barnowski <e9fb290bfd0a4406872fd6103425d9efafcfa051@berkeley.edu>
jakirkham/numpy,mattip/numpy,charris/numpy,numpy/numpy,mhvk/numpy,mattip/numpy,rgommers/numpy,mhvk/numpy,numpy/numpy,seberg/numpy,endolith/numpy,charris/numpy,pdebuyl/numpy,seberg/numpy,endolith/numpy,mattip/numpy,jakirkham/numpy,rgommers/numpy,mhvk/numpy,pdebuyl/numpy,numpy/numpy,jakirkham/numpy,seberg/numpy,anntzer/numpy,anntzer/numpy,mhvk/numpy,mattip/numpy,numpy/numpy,charris/numpy,endolith/numpy,pdebuyl/numpy,seberg/numpy,charris/numpy,endolith/numpy,pdebuyl/numpy,rgommers/numpy,rgommers/numpy,anntzer/numpy,mhvk/numpy,jakirkham/numpy,anntzer/numpy,jakirkham/numpy