[llvm] [llvm][test][NFCI] Use absolute build path instead of a relative path (PR #159126)

[Michał Górny via llvm-commits]

mgorny wrote:

This path still seems wrong:

```
# | Error opening '/var/tmp/portage/llvm-core/llvm-22.0.0.9999/work/llvm_build-.arm64/lib64/unittests/CodeGen/CGPluginTest/CGTestPlugin.so': /var/tmp/portage/llvm-core/llvm-22.0.0.9999/work/llvm_build-.arm64/lib64/unittests/CodeGen/CGPluginTest/CGTestPlugin.so: cannot open shared object file: No such file or directory
```

I'm seeing:

```
# find /var/tmp/portage/llvm-core/llvm-22.0.0.9999/work/llvm_build-.arm64 -name CGTestPlugin.so
/var/tmp/portage/llvm-core/llvm-22.0.0.9999/work/llvm_build-.arm64/lib64/CGTestPlugin.so
```

https://github.com/llvm/llvm-project/pull/159126