Adjust test setup to work inside a docker executor

[Clockwork-Muse]

I'm currently experimenting with building my own copy of the images (as I'm using/building panda3d), but my CI system uses a docker executor (and builds the image with docker-in-docker), and thus it fails when tests are run because build.sh assumes that the current working directory is a host directory, and volume-mounts the test directory.

I would appreciate it if the way tests were run were adjusted to account for this. Volume-busting is unlikely to be a good solution; the only thing likely to work in most cases is building a sub image that copies the test directory. Failing that, a way to stop the tests from running would also work in my case.