docs: add test badge on README

Add GitHub Actions test badge to showcase the project's test status directly on the README.md file. This helps users quickly understand the project's build status and test reliability.

Signed-off-by: Avelino <[email protected]>

Author: avelino

README.md

@@ -2,6 +2,8 @@
 
 An **unofficial** Python SDK for Santander's API that simplifies integration with Santander banking services.
 
+[![test](https://github.com/buserbrasil/santander-python-sdk/actions/workflows/test.yml/badge.svg)](https://github.com/buserbrasil/santander-python-sdk/actions/workflows/test.yml)
+
 ## Features
 
 - ✨ **Authentication**: Automatic token management