document testing strategy, patterns, frameworks

[jspaaks]

in README.dev.rst. Probably a good idea make an outline first befre filling in the details. May warrant its own document.

[jspaaks]

Tree below from https://github.com/fair-software/howfairis/pull/285#issue-576511440

Here is the new file structure for a given case:

.
├── github
│   ├── conftest.py
│   ├── frontend-files
│   │   └── index.html
│   ├── __init__.py
│   ├── snippets
│   │   └── cli-no-args.txt
│   ├── test_checker_*.py
│   ├── test_cli_*.py
│   └── test_repo_*.py
├── gitlab
│   ├── conftest.py
│   ├── frontend-files
│   │   └── index.html
│   ├── __init__.py
│   ├── snippets
│   │   └── cli-no-args.txt
│   ├── test_checker_*.py
│   ├── test_cli_*.py
│   └── test_repo_*.py
├── __init__.py
├── repo-files
│   ├── CITATION
│   ├── CITATION.cff
│   ├── codemeta.json
│   ├── .howfairis.yml
│   ├── README.rst
│   └── .zenodo.json
└── user-files
    ├── .howfairis-default.yml
    ├── .howfairis-emptyreasons.yml
    └── .howfairis-skipreasons.yml