test: introduce TEST_PARALLELIZE to support running tests in parallel

Support running tests in parallel by switching to copying of the
base image instead of symlinking it..

This still requires some setup steps, like running `make setup` on tests
which have unique $IMAGE_NAME beforehand (and sequentially), otherwise
they'll all try to create the same base image when started in parallel,
leading to nasty issues. However, as running the integration tests in
parallel is such an unusual use case it should be good enough, for now.