feat: add test status badge to README (#57)

README.md

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 # Eppo .NET SDK
 
+[![Run Tests](https://github.com/Eppo-exp/dot-net-server-sdk/actions/workflows/run-tests.yml/badge.svg)](https://github.com/Eppo-exp/dot-net-server-sdk/actions/workflows/run-tests.yml)
+
+
 [Eppo](https://www.geteppo.com/) is a modular flagging and experimentation analysis tool. Eppo's .NET SDK is built to make assignments in multi-user server side contexts, compatible with Dot Net 7.0 Runtime. Before proceeding you'll need an Eppo account.
 
 Despite the name, this SDK runs in both server and client applications.