[llvm] [GSYM] Update gSym unit test with stable / portable path (PR #131204)

[Fangrui Song via llvm-commits]

MaskRay wrote:

>  the test would significantly change every time it was regenerated due to the generation script using a temporary directory as the compilation directory.

Thanks for the patch! This is not the root cause. Different developers might have the build directory at different locations, so even  if the script does not use a temporary directory, the directory will be different if the test is regenerated by another developer.

https://github.com/llvm/llvm-project/pull/131204