Move src/test to the root
See MCP at rust-lang/compiler-team#573
There may be more changes needed.
The first commit is just the move of the files:
You can check that the first commit did not do anything else than renames by running
```
git diff --diff-filter=r -M100% <rust-lang remote>/master <first commit hash>
```
The output should be empty, because the filter excludes renames, and the match threshold for qualifying a rename is 100%.
The second one is mostly a "find and replace" of `src/test` to `tests` and whatever is needed to make CI pass.
What is left to do:
---
- [x] Move directory
- [ ] Change references to `src/test`
- [x] Change references in-tree
- [ ] Change references in submodules / out-of-tree docs
- [x] Make CI pass:
- [x] Fix tidy
- [x] Fix tests
- [x] Bless tests if needed (shouldn't normally)
- [ ] Merge it !
b22c152958