Fix dev=yes in unit testing guide

[catwasd]

Description

This PR fixes the outdated usage of dev=yes in contributor guide.

Tests are compiled automatically if you use the dev=yes SCons option. dev=yes is recommended if you plan on contributing to the engine development as it will automatically treat compilation warnings as errors. The continuous integration system will fail if any compilation warnings are detected, so you should strive to fix all warnings before opening a pull request.

Link: https://docs.godotengine.org/en/stable/contributing/development/core_and_modules/unit_testing.html

PR that changed the option: godotengine/godot#66242

It is found in 4.0, 4.1, latest, and stable.

[mhilbrunner]

Thanks! Merged. Sorry this has been sitting for a bit without response :)