check the static path for build and install locations

allows packages to run the tests during package build and install time
where the two file locations might be split.
use DEFAULT_STATIC_FILES_PATH to find css files

get_ipython_package_dir() does not necessarily contain the files, e.g.
in distribution packages which share files with python3 in a different
folder.
Merge pull request #4868 from juliantaylor/static-path-fixes

Static path fixes