Refer to Git exercise in the intro to testing

Author: IshaanDesai

05_testing_and_ci/intro_slides.md

@@ -43,6 +43,7 @@ slideOptions:
 - Smelling old milk before using it!
 - A method to ensure that a software is producing reliable results.
 - Manual testing vs. automated testing.
+- You already saw tests in the [Git exercise](https://github.com/Simulation-Software-Engineering/git-exercise).
 
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