test: add coverage for preservePath - #1454

Closes #1453.

preservePath had no test coverage — grep -r preservePath test/ returned nothing — even though it changes what consumers see in file.originalname. This adds the two cases described in the issue:

• default (preservePath unset): a file sent as filename="a/b/c.txt" arrives with originalname === 'c.txt'
• preservePath: true: the same upload keeps originalname === 'a/b/c.txt'

Both go through util.submitForm like the existing tests. The full client path is set with form-data's filepath option, since filename is passed through path.basename internally and so can't express the path segments this option exists to preserve.

Verified the assertions actually exercise the option: forcing preservePath: false in lib/make-middleware.js fails the second case with -c.txt / +a/b/c.txt, and it passes again once reverted.

npm test goes from 84 to 86 passing, and npm run lint is clean. No source or existing test files are touched.