Consider how to replace CARGO_MANIFEST_DIR without trailing slash

https://github.com/dtolnay/trybuild/issues/202#issuecomment-1284634512

> My macro outputs "Help: Searching in directory $CARGO_MANIFEST_DIR" when the file it was looking for doesn't exist, which by its nature, will not always be the same path. However, trybuild for some reason doesn't replace that directory with $DIR but leaves the path untouched.

Start with the linked issue comment and inspect trybuild's diagnostic path replacement behavior for CARGO_MANIFEST_DIR and $DIR. Reproduce a missing-file diagnostic where the manifest directory has no trailing slash, then determine the expected normalized output and verify both trailing-slash cases are covered.

Written by the indexing model from the issue text.