[clang] [CLANG][NFC] Modify test cases to suit assigned default sysroot path (PR #77075)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> @MaskRay Wasn't it you that mentioned this?

Yes. I hoped that we could deprecated `DEFAULT_SYSROOT`: https://reviews.llvm.org/D158218 , but the relative `DEFAULT_SYSROOT` does not have a good replacement.

https://github.com/llvm/llvm-project/pull/77075