chore: Add tests and coverage workflow status badge

The badge should be visible once the workflow ran for the first time.

README.md

@@ -1,5 +1,7 @@
 # RDFProxy
 
+![tests](https://github.com/acdh-oeaw/rdfproxy/actions/workflows/tests.yaml/badge.svg)
+[![coverage](https://coveralls.io/repos/github/acdh-oeaw/rdfproxy/badge.svg?branch=main&kill_cache=1)](https://coveralls.io/github/acdh-oeaw/rdfproxy?branch=main&kill_cache=1)
 [![License: GPL v3](https://img.shields.io/badge/License-GPLv3-blue.svg)](https://www.gnu.org/licenses/gpl-3.0)
 [![Ruff](https://img.shields.io/endpoint?url=https://raw.githubusercontent.com/astral-sh/ruff/main/assets/badge/v2.json)](https://github.com/astral-sh/ruff)