Update tests for installing nbextensions
Update tests for installing nbextensions

jupyter/nbgrader,jupyter/nbgrader,EdwardJKim/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,jhamrick/nbgrader,EdwardJKim/nbgrader,ellisonbg/nbgrader,ellisonbg/nbgrader,jhamrick/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,dementrock/nbgrader,EdwardJKim/nbgrader,ellisonbg/nbgrader,EdwardJKim/nbgrader,ellisonbg/nbgrader,jupyter/nbgrader,dementrock/nbgrader