How to provide `testlib.h` for checker in SandboxFusion

[srijan-bansal]

I am trying to use the checker field from this dataset: [Code-Contests-Plus on Hugging Face](https://huggingface.co/datasets/ByteDance-Seed/Code-Contests-Plus) to compare the expected and actual outputs in SandboxFusion.

However, when I include #include "testlib.h" in my checker, the execution fails with:

testlib.h: No such file or directory

I understand that SandboxFusion does not ship testlib.h by default. Could you clarify the correct way to provide testlib.h to the sandbox execution?

[srijan-bansal]

@ssine @chenhaiq Can you please take a look at this