[DOC] Document how to run tests

[fabianegli]

Developers need to run tests locally and it would be nice to have documentation about how to run the tests in this repo during package development.

[fabianegli]

Ideally a pre-commit hook would be provided.

I was two types of tests in the CI, one integration test that converts a file and looks if the converted file matches the expected file and one that runs the tests in sdrf_pipelines/tests/sdrfchecker_tests.py. pythonapp.yml and
pythonpackage.yml seem to be mostly identical and not to be active, i.e. not running upon push/merge.

[ypriverol]

This is also a really nice first task. Also, if we want to involve more developers in the project in the future and for us to come back every time is easier to have a better set of tests. 👍