Lint YAML files under test/

This commit extends YAML linting by enabling standard rules from the
`yamllint` tool [1]. Since syntax errors and key duplicates are already
checked since 4d48711, this change only adds detection for cosmetic
problems. It also narrows checks to the test/ dir only.

The main goal is to prevent future problems to enter the code base
without being noticed. While it would be a huge effort to be PEP8
compliant, it is relatively easy to have correct YAML style *now* and
prevent future errors by enabling linting.

Note: for those (like me) caring about code attribution: use `git blame
-w` to ignore whitespace-only changes.

Note: I disabled some linting checks (such as indentation), they can be
enforced in the future if needed. Similarly, current checks can also be
disabled. See the `.yamllint` file.

[1]: https://yamllint.readthedocs.io/

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