add bootstrap tests for detecting src and out

[jyn514]

I was thinking about the following cases when I wrote the comment in #109055

1. Running bootstrap from the source root.
2. Running from a subdirectory of the source root.
3. Running from outside the source root.
4. Running on a different machine from where bootstrap was compiled (which will be important for no python in shell scripts #107812). You can mostly replicate this by renaming the source root so it no longer exists on disk.
5. Running with --build-dir.
6. Running with $RUST_BOOTSTRAP_CONFIG set in the environment and build-dir set in the file.

Originally posted by @jyn514 in #109055 (comment)