[BrowserKit] added a test to make sure HTTP authentication is preserved

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no 
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

Since #6995 BrowseKit no longer seems to preserve the HTTP authentication when submitting a form. This PR adds a test to demonstrate the failure.