Refactor string formatting test cases to use pytest-cases

[jackirvine97]

Pytest-cases enables parametrisation of text fixtures and would allow (currently hardcoded) string formatting examples to be refactored into fixtures and used across multiple tests. Pytest-cases is presently not available on conda-forge, however this has been raised with maintainers: smarie/python-pytest-cases#135.

If pytest-cases is added to conda-forge the refactored tests are implemented in the repo at commit 3672345.

[brocksam]

This is now feasible. See: https://anaconda.org/conda-forge/pytest-cases.