Integrate the installation-test with the main page

This adapts part of @gvwilson's 5b65a346 (Pulling in setup files,
2014-12-16) to link to the setup instructions without the other [side
effects of the original commit][1].  I've left off the changes to the
instruction text, because those should be happening upstream in
git://tremily.us/swc-setup-installation-test.git to avoid future merge
conflicts.  I've left of the renames:

* setup/swc-installation-test-2.py → util/test-setup-all.py
* setup/swc-installation-test-1.py → util/test-setup-minimal.py

because they also introduce differences from the upstream repository
with unclear benefits.  Keeping all of the vendored code in the setup/
directory also makes it easier to figure out where you should submit
changes (setup/ changes should go to
git://tremily.us/swc-setup-installation-test.git).

[1]: https://github.com/swcarpentry/workshop-template/pull/83#issuecomment-67352811