Fix: Controller Test withBody()

[MGatner]

Description
The ControllerTestTrait "with___" methods use local property references to apply their changes before execution. This doesn't work for $body however because it is a string and not passed by reference, so using withBody() has no effect.

This PR moves the application of the body to the execute() method so it is properly applied to the request prior to execution.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide