Move tests of experimental GRPO with replay buffer to tests/experimental

[albertvillanova]

Move tests of experimental GRPO with replay buffer to tests/experimental, analogously as already done in:

• Move BCO tests to tests/experimental #4326

Follow-up to:

• [GRPO]: Sample from a Replay Buffer To Substitute Groups with 0 std. #4060

For context:

• Add support for testing experimental features #4082

[HuggingFaceDocBuilderDev]

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[qgallouedec]

Way better, thanks