[DomCrawler] Keep temporary file copy filenames

| Q             | A
| ------------- | ---
| Bug fix?      | kind of
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        |

DomCrawler's `FileFormField` takes a copy of the file and gives it a new filename (on Windows something like `upl8BF8.tmp`). Using Goutte I've tried to upload an image which is rejected by the server (in this case Drupal) since it no longer has a `.jpg` filename. This PR changes the copy so that it goes into a unique path and keeps its filename. (It also adds tests to make sure the copy is actually there.)