Cleanup the test suite and add dpkg_deploy test

Changes in this MR:

• Replaced the project's base platform with an import of an alpine image tarball as this is much quicker than the cumbersome freedesktop ostree import
• Moved all elements in the test suite up into the elements/ dir (instead of in subdirs)
  • With the exception of base/ which contains the elements required for the base.bst stack
• Simplified the tests
• Added a dpkg_deploy test
  • This required patching dpkg_deploy.yaml