tidy: split dots in filename not the entire path when checking for stray stdout/stderr files
I committed a path crime by splitting the entire path on `.`, when I meant to split on the filename. This means that any parent folders which contain `.` will cause tidy failure. Added a regression test so that doesn't happen again.
### Follow-up
- [ ] Adjust rustc-dev-guide to document assert on test name not containing dots. https://github.com/rust-lang/rustc-dev-guide/pull/1927Fixes#121986.
c7beecf3e3