Add hint how to use gradle test fixtures

[JanWiemer]

Add hint how to use gradle test fixtures to the getting-started-testing.adoc document

I tried to use gradle test fixtures with a Quarkus project and had some issues getting it to work. I asked in Quarkus chat and later posted my solution. There was the Idea to contribute a short hint in the documentation. Here it is :-)

See https://quarkusio.zulipchat.com/#narrow/channel/187030-users/topic/.E2.9C.94.20Quarkus.20dependencies.20and.20gradle.20test.20fixtures

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🎊 PR Preview 2c4d218d5da92988ac3e1bd0fd7e5039d1767342 has been successfully built and deployed to https://quarkus-pr-main-44555-preview.surge.sh/version/main/guides/

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