Auto merge of #109162 - ozkanonur:extend_detect_src_and_out_test, r=jyn514

extend `detect_src_and_out` test

> 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 #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.

Tested all the topics mentioned above. All worked fine. The test is now also covers if build dir is manually specified in config.

r? `@jyn514`

helps #109120 partially

Author: bors