Adds chapter 10 for the Dart GSE (#6775)

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Co-authored-by: Amanda Fitch <[email protected]>

Author: lamek

src/content/get-started/index.md

@@ -31,3 +31,4 @@ follow the instructions on how to [Get the Dart SDK][].
 7. [Advanced OOP](/get-started/advanced-oop)
 8. [Command_runner polish](/get-started/command-runner-polish)
 9. [Data and JSON](/get-started/data-and-json)
+10. [Testing](/get-started/testing)