You have created a user control. More information about user control, you can find in Sitefinity documentation:
If you look in your HelloWorldTest.cs file, you will find a field referencing the layout template path:
which is used in LayoutTemplatePath property:
.LayoutTemplatePath = value;
You can also consider watch this nice video tutorial made by Gabe Sumners for User Controls and Control Designers in Sitefinity:
I hope this helps.
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