test: adding badge for a top-level status report

tests/README.md

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+# Test report
+
+- overall status: ![CI test build](https://github.com/sdsc-ordes/saving-willy/actions/workflows/python-pytest.yml/badge.svg)
+- more detailed test report: TODO
+
+## Test coverage
+
+- TODO
+   - For a summary: one way is using https://github.com/GaelGirodon/ci-badges-action, can add it as a post-pytest step to the CI
+   - For a table: try this https://github.com/coroo/pytest-coverage-commentator