The current `test/test_helper/common.bash` was getting large. Setup logic has been extracted out into a new file.
`common.bash` resides in a directory named `test_helper/`, the `test_` prefix is redundant.
As an interim solution this provides a new approach for the updated tests, while the "old" tests can use the "old" `common.bash`. Eventually all tests should migrate to the new approach in `helper/` instead of the older `test_helper/`.
The new helper files are located under `test/helper/` (_which drops the `test_` prefix_). The new and updated helpers apply the new naming convention for ENV variables (_such as `CONTAINER_NAME` or `IMAGE_NAME`_).
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Some refactoring occurred, including new methods like `_run_in_container()` and `_default_teardown()`.
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I encountered a situation before in which the updated tests would fail because there were collisions of ENV names in the tests (_for example with `CONTAINER_NAME`_).
