[FT] Add tests for `VLLMModel` base methods

[NathanHB]

Issue encountered

From @lewtun: in addition to adding integration tests for the vllm backend of lighteval WDYT about prioritising some unit tests for the VLLMModel class methods? That would help catch subtle bugs like this and also protect us a bit from silent, yet breaking changes on the vllm side

Solution/Feature

Add tests for VLLMModel