Move test-stack code to its own bundle

[gggeek]

The code allowing to run tests both on Travis and in Docker keeps growing. It is now refined enough and generic enough to be useable by other bundles that share the same requirement of being tested across many version of eZP. It makes thus sense to split it out into its own package.

[gggeek]

The recent changes to the policy of free-support-for-open-source-projects from Travis make this all the more important. Given that transitioning to another CI-as-a-service platform seems risky (atm Github Action is trending, but it offers limited free monthly minutes), we should strive to be able to easily run locally the full suite of tests across the whole matrix of php versions / environments.

Without adopting a separate, existing CI tool, it should not be too hard to:

• add support for PostreSQL as an alternative to mysql
• add to teststack.sh the possibility of running the tests (sequentially) on a matrix of envs instead of a single one

[gggeek]

Done - as of today the test-stack code lives in its own bundle at https://github.com/tanoconsulting/euts.