add test badge

Author: sophiamaedler

README.md

@@ -4,6 +4,8 @@
 [![Docs](https://img.shields.io/website?url=https%3A%2F%2Fmannlabs.github.io/scPortrait/index.html)](https://mannlabs.github.io/scPortrait/index.html)
 [![PyPI version](https://img.shields.io/pypi/v/scportrait.svg)](https://pypi.org/project/scportrait/)
 [![PyPI Downloads](https://static.pepy.tech/badge/scportrait)](https://pepy.tech/projects/scportrait)
+[![Unit Tests](https://github.com/MannLabs/scPortrait/actions/workflows/unit_test.yml/badge.svg)](https://github.com/MannLabs/scPortrait/actions/workflows/unit_test.yml)
+<!-- [![E2E Tests](https://github.com/MannLabs/scPortrait/actions/workflows/e2e_test.yml/badge.svg)](https://github.com/MannLabs/scPortrait/actions/workflows/e2e_test.yml)-->
 
 
 ![graphical_abstract](https://github.com/user-attachments/assets/02d07c03-243f-4468-8e77-09d703111d11)