Changes to test_helpers.py from astropy/astropy#2353, astropy/astropy#2214, and astropy/astropy#2094
Changes to test_helpers.py from astropy/astropy#2353, astropy/astropy#2214, and astropy/astropy#2094

bsipocz/astropy-helpers,larrybradley/astropy-helpers,larrybradley/astropy-helpers,embray/astropy_helpers,embray/astropy_helpers,Cadair/astropy-helpers,bsipocz/astropy-helpers,embray/astropy_helpers,embray/astropy_helpers,bsipocz/astropy-helpers,larrybradley/astropy-helpers,Cadair/astropy-helpers,astropy/astropy-helpers,dpshelio/astropy-helpers,dpshelio/astropy-helpers,astropy/astropy-helpers