Add integration tests for metapackages

This MR implements a system for defining integration tests for each metapackage. The package YAML files can now contain a test list that can include commands executed by bash after the packages are installed, to verify that they work and the dependencies are well formatted, e.g.:

test:
  - python -c "import numpy"

ATOW this exposes the bug fixed in !21 (merged) that means most of the metapackage dependencies aren't declared properly for debian.

@adam-mercer, once !21 (merged) is merged I will rebase this and we can see how well it works.