4.x: Add documentation for @Testing.Test

[romain-grecourt]

Helidon Version: > 4.2.x
Flavor: SE

Enhancement Description

The global setter methods are deprecated in favor of the global registry (via io.helidon.service.registry.Services).

@Testing.Test is provided to setup a dedicated global registry per test and thus isolate tests from each others.

We must document @Testing.Test to help users update their usasges of the global setters in their unit tests.