[libc++][test] Improves src include paths. (#86864)
Instead of including a relative path use an absolute path based on the
available lit substitution. This makes it easier to understand what is
included and moving the test to a different directory level no longer
breaks the test.
This is based on a question by @EricWF in
https://github.com/llvm/llvm-project/pull/82113.
