TST: Add test for astype to stringlength tests
TST: Add test for astype to stringlength tests

astype is similar to array coercion, and is also compared in
many of the tests below.

jakirkham/numpy,numpy/numpy,pbrod/numpy,rgommers/numpy,mattip/numpy,anntzer/numpy,pdebuyl/numpy,mattip/numpy,madphysicist/numpy,madphysicist/numpy,grlee77/numpy,pbrod/numpy,seberg/numpy,simongibbons/numpy,mhvk/numpy,seberg/numpy,grlee77/numpy,pdebuyl/numpy,charris/numpy,anntzer/numpy,madphysicist/numpy,seberg/numpy,seberg/numpy,endolith/numpy,numpy/numpy,endolith/numpy,mattip/numpy,charris/numpy,numpy/numpy,pdebuyl/numpy,mhvk/numpy,anntzer/numpy,grlee77/numpy,rgommers/numpy,madphysicist/numpy,numpy/numpy,pbrod/numpy,pdebuyl/numpy,simongibbons/numpy,mattip/numpy,grlee77/numpy,simongibbons/numpy,jakirkham/numpy,pbrod/numpy,madphysicist/numpy,mhvk/numpy,mhvk/numpy,pbrod/numpy,charris/numpy,charris/numpy,simongibbons/numpy,jakirkham/numpy,endolith/numpy,jakirkham/numpy,endolith/numpy,rgommers/numpy,grlee77/numpy,simongibbons/numpy,mhvk/numpy,anntzer/numpy,jakirkham/numpy,rgommers/numpy